docs: add FORK.md with feature summary

- Document all security features added
- Document UX improvements added
- Include migration strategy and backward compatibility notes
- Provide enable instructions for optional features

FORK.md

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+# Fork Summary
+
+This fork adds optional security features and UX improvements with **zero breaking changes** and **minimal migration overhead**. All security features are **disabled by default** via feature flags.
+
+## Security Features Added
+
+1. **Password Reset** - Token-based password reset flow (`/auth/password-reset-request`, `/auth/password-reset-confirm`)
+2. **Refresh Token Rotation** - Prevents token reuse by rotating refresh tokens on each use
+3. **Audit Logging** - Logs security events (logins, password changes, deletions) for compliance
+
+## UX Improvements Added
+
+1. **Profile Page** - View and edit personal information, change password (`/profile`)
+2. **Select All Button** - Quick selection of all drawings in current view
+3. **Sort Dropdown** - Improved sort controls with icons and separate direction toggle
+4. **Auto-hide Header** - Editor header auto-hides to maximize drawing space (with toggle)
+
+## Backward Compatibility
+
+✅ All security features disabled by default  
+✅ No breaking changes to existing code  
+✅ Graceful degradation (missing tables don't cause errors)  
+✅ Optional database migration  
+
+## Enable Security Features
+
+Set in `backend/.env`:
+```bash
+ENABLE_PASSWORD_RESET=true
+ENABLE_REFRESH_TOKEN_ROTATION=true
+ENABLE_AUDIT_LOGGING=true
+```
+
+Then run migration:
+```bash
+cd backend && npx prisma migrate deploy
+```
+
+## Migration Strategy
+
+**For base project:** Keep features disabled (default) - no migration needed, zero risk.
+
+**For this fork:** Enable features via environment variables when ready.
+
+## Database Changes
+
+Migration adds 3 optional tables (only used when features enabled):
+- `PasswordResetToken` - For password reset flow
+- `RefreshToken` - For token rotation tracking
+- `AuditLog` - For security event logging
+
+## Code Changes
+
+### Backend
+- Feature flags in `backend/src/config.ts`
+- Conditional logic in auth endpoints
+- Graceful error handling for missing tables
+- New endpoints: `/auth/profile` (PUT), `/auth/change-password` (POST)
+- Audit logging utility (`backend/src/utils/audit.ts`)
+
+### Frontend
+- Password reset pages (`/reset-password`, `/reset-password-confirm`)
+- Profile page (`/profile`)
+- Select All button in Dashboard
+- Sort dropdown with icons
+- Auto-hide header in Editor with toggle
+- Updated API client for token rotation
+
+All changes are backward compatible and optional.