feat(collab): add server-authoritative sync and preview-only updates

backend/src/index.ts

@@ -16,7 +16,7 @@ import {
   sanitizeDrawingData,
   validateImportedDrawing,
   sanitizeText,
-  sanitizeSvg,
+  sanitizePreview,
   elementSchema,
   appStateSchema,
 } from "./security";
@@ -29,6 +29,7 @@ import { registerDashboardRoutes } from "./routes/dashboard";
 import { registerImportExportRoutes } from "./routes/importExport";
 import { prisma } from "./db/prisma";
 import { createDrawingsCacheStore } from "./server/drawingsCache";
+import { createCollabSessionManager } from "./server/collabSession";
 import { registerCsrfProtection } from "./server/csrf";
 import { registerSocketHandlers } from "./server/socket";
 import { getClientIp } from "./utils/clientIp";
@@ -173,6 +174,7 @@ const {
   cacheDrawingsResponse,
   invalidateDrawingsCache,
 } = createDrawingsCacheStore(DRAWINGS_CACHE_TTL_MS);
+const collabSessionManager = createCollabSessionManager({ prisma });
 
 const getUserTrashCollectionId = (userId: string): string => `trash:${userId}`;
 
@@ -430,7 +432,7 @@ export const sanitizeDrawingUpdateData = (
       Object.assign(data, sanitizedData);
     } else if (hasPreviewField && typeof data.preview === "string") {
       // Preview-only updates must not inject default scene fields.
-      data.preview = sanitizeSvg(data.preview);
+      data.preview = sanitizePreview(data.preview);
       Object.assign(data, { ...data, preview: data.preview });
     } else if (hasPreviewField && data.preview === null) {
       // Explicitly allow clearing preview without touching scene data.
@@ -564,6 +566,7 @@ registerSocketHandlers({
   prisma,
   authModeService,
   jwtSecret: config.jwtSecret,
+  collabSessionManager,
 });
 
 apiApp.get("/health", (req, res) => {
@@ -589,6 +592,7 @@ registerDashboardRoutes(apiApp, {
   buildDrawingsCacheKey,
   getCachedDrawingsBody,
   cacheDrawingsResponse,
+  collabSessionManager,
   MAX_PAGE_SIZE,
   config,
   logAuditEvent,

backend/src/routes/dashboard/drawings.ts

@@ -58,6 +58,7 @@ export const registerDrawingRoutes = (
     buildDrawingsCacheKey,
     getCachedDrawingsBody,
     cacheDrawingsResponse,
+    collabSessionManager,
     MAX_PAGE_SIZE,
     config,
     logAuditEvent,
@@ -446,6 +447,57 @@ export const registerDrawingRoutes = (
     });
   }));
 
+  app.get("/drawings/:id/scene-meta", requireAuth, asyncHandler(async (req, res) => {
+    if (!req.user) return res.status(401).json({ error: "Unauthorized" });
+    const id = getRouteIdParam(req.params.id);
+    if (!id) return res.status(400).json({ error: "Validation error", message: "Invalid id parameter" });
+
+    const access = await resolveDrawingAccess({
+      prisma,
+      drawingId: id,
+      userId: req.user.id,
+    });
+    if (!access) return res.status(404).json({ error: "Drawing not found" });
+
+    const meta = await collabSessionManager.getMeta(id);
+    if (!meta) {
+      return res.status(404).json({ error: "Drawing not found" });
+    }
+
+    return res.json({
+      drawingId: id,
+      seq: meta.seq,
+      dbVersion: meta.dbVersion,
+      updatedAt: access.drawing.updatedAt,
+    });
+  }));
+
+  app.post("/drawings/:id/flush", requireAuth, asyncHandler(async (req, res) => {
+    if (!req.user) return res.status(401).json({ error: "Unauthorized" });
+    const id = getRouteIdParam(req.params.id);
+    if (!id) return res.status(400).json({ error: "Validation error", message: "Invalid id parameter" });
+
+    const access = await resolveDrawingAccess({
+      prisma,
+      drawingId: id,
+      userId: req.user.id,
+    });
+    if (!access) return res.status(404).json({ error: "Drawing not found" });
+    if (!isAtLeastRole(access.role, "editor")) {
+      return res.status(403).json({ error: "Forbidden", message: "You do not have edit access" });
+    }
+
+    await collabSessionManager.flushSession(id);
+    const meta = await collabSessionManager.getMeta(id);
+
+    return res.json({
+      success: true,
+      drawingId: id,
+      seq: meta?.seq ?? null,
+      dbVersion: meta?.dbVersion ?? null,
+    });
+  }));
+
   app.post("/drawings", requireAuth, asyncHandler(async (req, res) => {
     if (!req.user) return res.status(401).json({ error: "Unauthorized" });
 
@@ -630,6 +682,50 @@ export const registerDrawingRoutes = (
     });
   }));
 
+  app.put("/drawings/:id/preview", requireAuth, asyncHandler(async (req, res) => {
+    if (!req.user) return res.status(401).json({ error: "Unauthorized" });
+
+    const id = getRouteIdParam(req.params.id);
+    if (!id) {
+      return res.status(400).json({ error: "Validation error", message: "Invalid id parameter" });
+    }
+
+    const access = await resolveDrawingAccess({
+      prisma,
+      drawingId: id,
+      userId: req.user.id,
+    });
+    if (!access) return res.status(404).json({ error: "Drawing not found" });
+    if (!isAtLeastRole(access.role, "editor")) {
+      return res.status(403).json({ error: "Forbidden", message: "You do not have edit access" });
+    }
+
+    const parsed = drawingUpdateSchema.safeParse({ preview: req.body?.preview });
+    if (!parsed.success) {
+      if (config.nodeEnv === "development") {
+        console.error("[API] Preview validation failed", { id, errors: parsed.error.issues });
+      }
+      return respondWithValidationErrors(res, parsed.error.issues);
+    }
+
+    const payload = parsed.data as { preview?: string | null };
+    if (payload.preview === undefined) {
+      return res.status(400).json({ error: "Validation error", message: "Missing preview field" });
+    }
+
+    const updated = await prisma.drawing.updateMany({
+      where: access.role === "owner" ? { id, userId: req.user.id } : { id },
+      data: { preview: payload.preview },
+    });
+
+    if (updated.count === 0) {
+      return res.status(404).json({ error: "Drawing not found" });
+    }
+
+    invalidateDrawingsCache();
+    return res.json({ success: true });
+  }));
+
   app.delete("/drawings/:id", requireAuth, asyncHandler(async (req, res) => {
     if (!req.user) return res.status(401).json({ error: "Unauthorized" });
     const id = getRouteIdParam(req.params.id);

backend/src/routes/dashboard/types.ts

@@ -2,6 +2,7 @@ import express from "express";
 import { z } from "zod";
 import { Prisma, PrismaClient } from "../../generated/client";
 import { AuthModeService } from "../../auth/authMode";
+import { CollabSessionManager } from "../../server/collabSession";
 
 export type SortField = "name" | "createdAt" | "updatedAt";
 export type SortDirection = "asc" | "desc";
@@ -48,6 +49,7 @@ export type DashboardRouteDeps = {
   buildDrawingsCacheKey: BuildDrawingsCacheKey;
   getCachedDrawingsBody: (key: string) => Buffer | null;
   cacheDrawingsResponse: (key: string, payload: unknown) => Buffer;
+  collabSessionManager: CollabSessionManager;
   MAX_PAGE_SIZE: number;
   config: {
     nodeEnv: string;

backend/src/security.ts

@@ -231,6 +231,31 @@ export const sanitizeSvg = (svgContent: string): string => {
   return sanitizeSvgImageTags(sanitized).trim();
 };
 
+const SAFE_PREVIEW_DATA_URL_PATTERN =
+  /^data:image\/(?:webp|png|jpe?g);base64,[a-z0-9+/=\s]+$/i;
+const MAX_PREVIEW_SIZE = 300_000;
+
+export const sanitizePreview = (
+  previewContent: string | null | undefined
+): string | null => {
+  if (previewContent === null || previewContent === undefined) return null;
+  if (typeof previewContent !== "string") return null;
+  const trimmed = previewContent.trim();
+  if (trimmed.length === 0) return null;
+  if (trimmed.length > MAX_PREVIEW_SIZE) return null;
+
+  if (SAFE_PREVIEW_DATA_URL_PATTERN.test(trimmed)) {
+    return trimmed;
+  }
+
+  if (/^<svg[\s>]/i.test(trimmed)) {
+    const sanitized = sanitizeSvg(trimmed);
+    return sanitized.length > 0 && sanitized.length <= MAX_PREVIEW_SIZE ? sanitized : null;
+  }
+
+  return null;
+};
+
 export const sanitizeText = (
   input: unknown,
   maxLength: number = 1000
@@ -463,10 +488,7 @@ export const sanitizeDrawingData = (data: {
     const sanitizedElements = elementSchema.array().parse(data.elements);
     const sanitizedAppState = appStateSchema.parse(data.appState);
 
-    let sanitizedPreview = data.preview;
-    if (typeof sanitizedPreview === "string") {
-      sanitizedPreview = sanitizeSvg(sanitizedPreview);
-    }
+    const sanitizedPreview = sanitizePreview(data.preview);
 
     // Sanitize files object with special handling for dataURL
     let sanitizedFiles = data.files;

backend/src/server/collabSession.ts

@@ -0,0 +1,293 @@
+import { PrismaClient } from "../generated/client";
+
+type SceneOp = {
+  upsertElements?: any[];
+  deleteElementIds?: string[];
+  filesDelta?: Record<string, any>;
+  appStatePatch?: Record<string, unknown>;
+};
+
+type DrawingSession = {
+  drawingId: string;
+  seq: number;
+  dbVersion: number;
+  elementsById: Map<string, any>;
+  filesById: Record<string, any>;
+  appState: Record<string, unknown>;
+  dirty: boolean;
+  lastTouchedAt: number;
+  participants: Set<string>;
+  pendingFlushTimer: NodeJS.Timeout | null;
+};
+
+type CreateCollabSessionManagerDeps = {
+  prisma: PrismaClient;
+  flushDebounceMs?: number;
+  idleEvictMs?: number;
+};
+
+const ALLOWED_APPSTATE_KEYS = new Set([
+  "viewBackgroundColor",
+  "gridSize",
+  "zoom",
+  "scrollX",
+  "scrollY",
+  "theme",
+]);
+
+const parseJsonSafely = <T>(rawValue: string | null | undefined, fallback: T): T => {
+  if (!rawValue) return fallback;
+  try {
+    return JSON.parse(rawValue) as T;
+  } catch {
+    return fallback;
+  }
+};
+
+const sanitizeElements = (elements: unknown): any[] => {
+  if (!Array.isArray(elements)) return [];
+  return elements.filter((element) => element && typeof element === "object");
+};
+
+const sanitizeDeleteIds = (deleteElementIds: unknown): string[] => {
+  if (!Array.isArray(deleteElementIds)) return [];
+  return deleteElementIds.filter((value): value is string => typeof value === "string");
+};
+
+const sanitizeFilesDelta = (filesDelta: unknown): Record<string, any> => {
+  if (!filesDelta || typeof filesDelta !== "object") return {};
+  const next: Record<string, any> = {};
+  for (const [key, value] of Object.entries(filesDelta as Record<string, unknown>)) {
+    if (typeof key !== "string" || key.length === 0) continue;
+    next[key] = value;
+  }
+  return next;
+};
+
+const sanitizeAppStatePatch = (appStatePatch: unknown): Record<string, unknown> => {
+  if (!appStatePatch || typeof appStatePatch !== "object") return {};
+  const next: Record<string, unknown> = {};
+  for (const [key, value] of Object.entries(appStatePatch as Record<string, unknown>)) {
+    if (!ALLOWED_APPSTATE_KEYS.has(key)) continue;
+    next[key] = value;
+  }
+  return next;
+};
+
+export const createCollabSessionManager = ({
+  prisma,
+  flushDebounceMs = 2000,
+  idleEvictMs = 5 * 60 * 1000,
+}: CreateCollabSessionManagerDeps) => {
+  const sessions = new Map<string, DrawingSession>();
+  const seenClientOps = new Map<string, number>();
+
+  const ensureSession = async (drawingId: string): Promise<DrawingSession | null> => {
+    const existing = sessions.get(drawingId);
+    if (existing) {
+      existing.lastTouchedAt = Date.now();
+      return existing;
+    }
+
+    const drawing = await prisma.drawing.findUnique({ where: { id: drawingId } });
+    if (!drawing) return null;
+
+    const elements = sanitizeElements(parseJsonSafely(drawing.elements, []));
+    const session: DrawingSession = {
+      drawingId,
+      seq: 0,
+      dbVersion: drawing.version,
+      elementsById: new Map(elements.map((element) => [String(element.id), element])),
+      filesById: parseJsonSafely(drawing.files, {}),
+      appState: parseJsonSafely(drawing.appState, {}),
+      dirty: false,
+      lastTouchedAt: Date.now(),
+      participants: new Set(),
+      pendingFlushTimer: null,
+    };
+
+    sessions.set(drawingId, session);
+    return session;
+  };
+
+  const getSceneSnapshot = async (drawingId: string) => {
+    const session = await ensureSession(drawingId);
+    if (!session) return null;
+    return {
+      drawingId,
+      seq: session.seq,
+      dbVersion: session.dbVersion,
+      elements: Array.from(session.elementsById.values()),
+      appState: session.appState,
+      files: session.filesById,
+    };
+  };
+
+  const flushSession = async (drawingId: string): Promise<boolean> => {
+    const session = sessions.get(drawingId);
+    if (!session) return false;
+    if (!session.dirty) return true;
+
+    if (session.pendingFlushTimer) {
+      clearTimeout(session.pendingFlushTimer);
+      session.pendingFlushTimer = null;
+    }
+
+    const elements = Array.from(session.elementsById.values());
+    await prisma.drawing.update({
+      where: { id: drawingId },
+      data: {
+        elements: JSON.stringify(elements),
+        appState: JSON.stringify(session.appState || {}),
+        files: JSON.stringify(session.filesById || {}),
+        version: { increment: 1 },
+      },
+    });
+
+    session.dbVersion += 1;
+    session.dirty = false;
+    session.lastTouchedAt = Date.now();
+    return true;
+  };
+
+  const scheduleFlush = (session: DrawingSession) => {
+    if (session.pendingFlushTimer) {
+      clearTimeout(session.pendingFlushTimer);
+    }
+    session.pendingFlushTimer = setTimeout(() => {
+      void flushSession(session.drawingId).catch((error) => {
+        console.error("[collab] failed to flush drawing session", {
+          drawingId: session.drawingId,
+          error,
+        });
+      });
+    }, flushDebounceMs);
+  };
+
+  const applyOps = async (params: {
+    drawingId: string;
+    clientOpId?: string;
+    ops: SceneOp[];
+  }): Promise<{ seq: number; duplicate: boolean } | null> => {
+    const session = await ensureSession(params.drawingId);
+    if (!session) return null;
+
+    const opKey = params.clientOpId ? `${params.drawingId}:${params.clientOpId}` : null;
+    if (opKey && seenClientOps.has(opKey)) {
+      return { seq: session.seq, duplicate: true };
+    }
+
+    for (const op of params.ops) {
+      const upsertElements = sanitizeElements(op?.upsertElements);
+      for (const element of upsertElements) {
+        const id = typeof element.id === "string" ? element.id : null;
+        if (!id) continue;
+        session.elementsById.set(id, element);
+      }
+
+      const deleteElementIds = sanitizeDeleteIds(op?.deleteElementIds);
+      for (const deleteId of deleteElementIds) {
+        const existing = session.elementsById.get(deleteId);
+        if (existing) {
+          session.elementsById.set(deleteId, { ...existing, isDeleted: true });
+        }
+      }
+
+      const filesDelta = sanitizeFilesDelta(op?.filesDelta);
+      if (Object.keys(filesDelta).length > 0) {
+        session.filesById = {
+          ...session.filesById,
+          ...filesDelta,
+        };
+      }
+
+      const appStatePatch = sanitizeAppStatePatch(op?.appStatePatch);
+      if (Object.keys(appStatePatch).length > 0) {
+        session.appState = {
+          ...session.appState,
+          ...appStatePatch,
+        };
+      }
+    }
+
+    session.seq += 1;
+    session.dirty = true;
+    session.lastTouchedAt = Date.now();
+    scheduleFlush(session);
+
+    if (opKey) {
+      seenClientOps.set(opKey, Date.now());
+    }
+
+    return { seq: session.seq, duplicate: false };
+  };
+
+  const joinSession = async (drawingId: string, socketId: string) => {
+    const session = await ensureSession(drawingId);
+    if (!session) return null;
+    session.participants.add(socketId);
+    session.lastTouchedAt = Date.now();
+    return session;
+  };
+
+  const leaveSession = async (drawingId: string, socketId: string): Promise<void> => {
+    const session = sessions.get(drawingId);
+    if (!session) return;
+    session.participants.delete(socketId);
+    session.lastTouchedAt = Date.now();
+
+    if (session.participants.size === 0) {
+      await flushSession(drawingId).catch((error) => {
+        console.error("[collab] failed to flush on last participant leave", {
+          drawingId,
+          error,
+        });
+      });
+    }
+  };
+
+  const getMeta = async (drawingId: string) => {
+    const session = await ensureSession(drawingId);
+    if (!session) return null;
+    return {
+      drawingId,
+      seq: session.seq,
+      dbVersion: session.dbVersion,
+      dirty: session.dirty,
+      lastTouchedAt: session.lastTouchedAt,
+    };
+  };
+
+  setInterval(() => {
+    const now = Date.now();
+
+    for (const [key, seenAt] of seenClientOps.entries()) {
+      if (now - seenAt > 10 * 60 * 1000) {
+        seenClientOps.delete(key);
+      }
+    }
+
+    for (const [drawingId, session] of sessions.entries()) {
+      const isIdle = now - session.lastTouchedAt > idleEvictMs;
+      if (!isIdle || session.participants.size > 0) continue;
+      void flushSession(drawingId)
+        .catch((error) => {
+          console.error("[collab] failed to flush idle session", { drawingId, error });
+        })
+        .finally(() => {
+          sessions.delete(drawingId);
+        });
+    }
+  }, 30_000).unref();
+
+  return {
+    getSceneSnapshot,
+    applyOps,
+    joinSession,
+    leaveSession,
+    flushSession,
+    getMeta,
+  };
+};
+
+export type CollabSessionManager = ReturnType<typeof createCollabSessionManager>;

backend/src/server/socket.ts

@@ -4,6 +4,7 @@ import { PrismaClient } from "../generated/client";
 import { AuthModeService } from "../auth/authMode";
 import { ACCESS_TOKEN_COOKIE_NAME, parseCookieHeader } from "../auth/cookies";
 import { isAtLeastRole, resolveDrawingAccess } from "./drawingAccess";
+import { CollabSessionManager } from "./collabSession";
 
 interface User {
   id: string;
@@ -19,6 +20,7 @@ type RegisterSocketHandlersDeps = {
   prisma: PrismaClient;
   authModeService: AuthModeService;
   jwtSecret: string;
+  collabSessionManager: CollabSessionManager;
 };
 
 export const registerSocketHandlers = ({
@@ -26,6 +28,7 @@ export const registerSocketHandlers = ({
   prisma,
   authModeService,
   jwtSecret,
+  collabSessionManager,
 }: RegisterSocketHandlersDeps) => {
   const roomUsers = new Map<string, User[]>();
   const socketUserMap = new Map<string, string>();
@@ -140,6 +143,7 @@ export const registerSocketHandlers = ({
 
           const roomId = `drawing_${drawingId}`;
           socket.join(roomId);
+          await collabSessionManager.joinSession(drawingId, socket.id);
 
           let trustedUserId =
             typeof user?.id === "string" && user.id.trim().length > 0
@@ -173,6 +177,11 @@ export const registerSocketHandlers = ({
           roomUsers.set(roomId, filteredUsers);
 
           io.to(roomId).emit("presence-update", filteredUsers);
+
+          const snapshot = await collabSessionManager.getSceneSnapshot(drawingId);
+          if (snapshot) {
+            socket.emit("scene-snapshot", snapshot);
+          }
         } catch (err) {
           console.error("Error in join-room handler:", err);
           socket.emit("error", { message: "Failed to join room" });
@@ -202,6 +211,45 @@ export const registerSocketHandlers = ({
       socket.to(roomId).emit("element-update", data);
     });
 
+    socket.on("scene-op", async (data) => {
+      const drawingId = typeof data?.drawingId === "string" ? data.drawingId : null;
+      if (!drawingId) return;
+
+      const role = authorizedDrawingRoles.get(drawingId);
+      if (!role || !isAtLeastRole(role, "editor")) {
+        return;
+      }
+
+      const baseSeq = typeof data?.baseSeq === "number" ? data.baseSeq : 0;
+      const clientOpId = typeof data?.clientOpId === "string" ? data.clientOpId : undefined;
+      const incomingOps = Array.isArray(data?.ops) ? data.ops : [];
+      if (incomingOps.length === 0) return;
+
+      const applied = await collabSessionManager.applyOps({
+        drawingId,
+        clientOpId,
+        ops: incomingOps,
+      });
+      if (!applied) return;
+
+      const roomId = `drawing_${drawingId}`;
+      io.to(roomId).emit("scene-op-applied", {
+        drawingId,
+        seq: applied.seq,
+        baseSeq,
+        ops: incomingOps,
+        authorUserId: authenticatedUserId ?? socket.id,
+        clientOpId,
+      });
+    });
+
+    socket.on("scene-flush", async (data) => {
+      const drawingId = typeof data?.drawingId === "string" ? data.drawingId : null;
+      if (!drawingId) return;
+      if (!authorizedDrawingRoles.has(drawingId)) return;
+      await collabSessionManager.flushSession(drawingId);
+    });
+
     socket.on(
       "user-activity",
       ({ drawingId, isActive }: { drawingId: string; isActive: boolean }) => {
@@ -222,6 +270,9 @@ export const registerSocketHandlers = ({
 
     socket.on("disconnect", () => {
       socketUserMap.delete(socket.id);
+      for (const drawingId of authorizedDrawingRoles.keys()) {
+        void collabSessionManager.leaveSession(drawingId, socket.id);
+      }
       roomUsers.forEach((users, roomId) => {
         const index = users.findIndex((u) => u.socketId === socket.id);
         if (index !== -1) {

frontend/src/api/index.ts

@@ -540,6 +540,44 @@ export const updateDrawing = async (id: string, data: Partial<Drawing>) => {
   return deserializeDrawing(response.data);
 };
 
+export const updateDrawingPreview = async (
+  id: string,
+  preview: string | null
+): Promise<{ success: true }> => {
+  const response = await api.put<{ success: true }>(`/drawings/${id}/preview`, { preview });
+  return response.data;
+};
+
+export const getDrawingSceneMeta = async (id: string): Promise<{
+  drawingId: string;
+  seq: number;
+  dbVersion: number;
+  updatedAt: string | number;
+}> => {
+  const response = await api.get<{
+    drawingId: string;
+    seq: number;
+    dbVersion: number;
+    updatedAt: string | number;
+  }>(`/drawings/${id}/scene-meta`);
+  return response.data;
+};
+
+export const flushDrawingScene = async (id: string): Promise<{
+  success: true;
+  drawingId: string;
+  seq: number | null;
+  dbVersion: number | null;
+}> => {
+  const response = await api.post<{
+    success: true;
+    drawingId: string;
+    seq: number | null;
+    dbVersion: number | null;
+  }>(`/drawings/${id}/flush`);
+  return response.data;
+};
+
 export const deleteDrawing = async (id: string) => {
   const response = await api.delete<{ success: true }>(`/drawings/${id}`);
   return response.data;

frontend/src/components/DrawingCard.tsx

@@ -7,7 +7,7 @@ import { formatDistanceToNow } from 'date-fns';
 import clsx from 'clsx';
 // import { exportToSvg } from "@excalidraw/excalidraw"; // Lazy load this instead
 import { exportDrawingToFile } from '../utils/exportUtils';
-import { previewHasEmbeddedImages } from '../utils/previewSvg';
+import { isPreviewImageDataUrl, previewHasEmbeddedImages } from '../utils/previewSvg';
 
 import * as api from '../api';
 
@@ -89,6 +89,7 @@ export const DrawingCard: React.FC<DrawingCardProps> = ({
   const [exportError, setExportError] = useState<string | null>(null);
   const [fullData, setFullData] = useState<HydratedDrawingData | null>(null);
   const hasEmbeddedImages = previewHasEmbeddedImages(previewSvg);
+  const isImagePreview = isPreviewImageDataUrl(previewSvg);
 
   const fullDataRef = React.useRef(fullData);
   fullDataRef.current = fullData;
@@ -161,7 +162,7 @@ export const DrawingCard: React.FC<DrawingCardProps> = ({
         setPreviewSvg(previewHtml);
 
         // Save to backend and notify parent
-        api.updateDrawing(drawing.id, { preview: previewHtml }).catch(console.error);
+        api.updateDrawingPreview(drawing.id, previewHtml).catch(console.error);
         onPreviewGenerated?.(drawing.id, previewHtml);
       } catch (e) {
         if (!cancelled) {
@@ -277,6 +278,16 @@ export const DrawingCard: React.FC<DrawingCardProps> = ({
           <div className="absolute inset-0 opacity-[0.3] bg-[linear-gradient(to_right,#80808012_1px,transparent_1px),linear-gradient(to_bottom,#80808012_1px,transparent_1px)] [background-size:24px_24px]"></div>
 
           {previewSvg ? (
+            isImagePreview ? (
+              <img
+                src={previewSvg}
+                alt=""
+                className="w-full h-full object-contain p-2 sm:p-3 lg:p-4 relative z-10"
+                loading="lazy"
+                decoding="async"
+                draggable={false}
+              />
+            ) : (
             <div
               className={clsx(
                 "w-full h-full p-3 sm:p-4 lg:p-5 flex items-center justify-center [&>svg]:w-auto [&>svg]:h-auto [&>svg]:max-w-full [&>svg]:max-h-full [&>svg]:drop-shadow-sm transition-transform duration-500",
@@ -284,6 +295,7 @@ export const DrawingCard: React.FC<DrawingCardProps> = ({
               )}
               dangerouslySetInnerHTML={{ __html: previewSvg }}
             />
+            )
           ) : (
             <div className="w-16 h-16 sm:w-20 sm:h-20 lg:w-24 lg:h-24 bg-white dark:bg-neutral-900 rounded-2xl shadow-sm flex items-center justify-center text-neutral-300 dark:text-neutral-400 border border-slate-100 dark:border-neutral-700 transform group-hover:scale-110 group-hover:rotate-3 transition-all duration-500">
               <PenTool size={32} strokeWidth={1.5} className="sm:w-9 sm:h-9 lg:w-10 lg:h-10" />

frontend/src/pages/Dashboard.tsx

@@ -11,6 +11,7 @@ import { ConfirmModal } from '../components/ConfirmModal';
 import { useUpload } from '../context/UploadContext';
 import { DragOverlayPortal, getSelectionBounds, type Point, type SelectionBounds } from './dashboard/shared';
 import { useDashboardData } from './dashboard/useDashboardData';
+import { isPreviewImageDataUrl } from '../utils/previewSvg';
 
 const PAGE_SIZE = 24;
 
@@ -699,10 +700,21 @@ export const Dashboard: React.FC = () => {
                 <div className="absolute inset-0 opacity-[0.3] bg-[linear-gradient(to_right,#80808012_1px,transparent_1px),linear-gradient(to_bottom,#80808012_1px,transparent_1px)] [background-size:24px_24px]"></div>
 
                 {d.preview ? (
+                  isPreviewImageDataUrl(d.preview) ? (
+                    <img
+                      src={d.preview}
+                      alt=""
+                      className="w-full h-full p-2 object-contain relative z-10"
+                      loading="lazy"
+                      decoding="async"
+                      draggable={false}
+                    />
+                  ) : (
                     <div
                       className="w-full h-full p-2 flex items-center justify-center [&>svg]:w-auto [&>svg]:h-auto [&>svg]:max-w-full [&>svg]:max-h-full [&>svg]:drop-shadow-sm relative z-10"
                       dangerouslySetInnerHTML={{ __html: d.preview }}
                     />
+                  )
                 ) : (
                   <div className="text-slate-300 relative z-10"><Folder size={24} /></div>
                 )}

frontend/src/utils/previewSvg.ts

@@ -65,11 +65,29 @@ export const previewHasEmbeddedImages = (
   preview: string | null | undefined
 ): boolean => typeof preview === "string" && /<image[\s>]/i.test(preview);
 
+export const isPreviewImageDataUrl = (
+  preview: string | null | undefined
+): boolean =>
+  typeof preview === "string" &&
+  /^data:image\/(?:webp|png|jpe?g);base64,[a-z0-9+/=\s]+$/i.test(preview.trim());
+
+export const isPreviewSvgMarkup = (
+  preview: string | null | undefined
+): boolean => typeof preview === "string" && /^\s*<svg[\s>]/i.test(preview);
+
 export const normalizePreviewSvg = (preview: string | null | undefined): string | null => {
   if (typeof preview !== "string" || preview.trim().length === 0) {
     return preview ?? null;
   }
 
+  if (isPreviewImageDataUrl(preview)) {
+    return preview.trim();
+  }
+
+  if (!isPreviewSvgMarkup(preview)) {
+    return null;
+  }
+
   if (typeof DOMParser === "undefined") {
     return preview;
   }