add register, login and main page with basic functionality

app/routes/auth.py

@@ -0,0 +1,57 @@
+from flask import Blueprint, render_template, request, redirect, url_for, session, flash
+from app.db import get_db
+from werkzeug.security import generate_password_hash, check_password_hash
+
+auth_bp = Blueprint("auth", __name__)
+
+@auth_bp.route("/login", methods=["GET", "POST"])
+def login():
+    if request.method == "POST":
+        username = request.form["username"]
+        password = request.form["password"]
+
+        db = get_db()
+        user = db.execute("SELECT * FROM users WHERE username = ?", (username,)).fetchone()
+
+        if user and check_password_hash(user["password"], password):
+            session["user_id"] = user["id"]
+            return redirect(url_for("main.main"))
+        else:
+            flash("Invalid username or password")
+
+    return render_template("login.html")
+
+
+@auth_bp.route("/register", methods=["GET", "POST"])
+def register():
+    if request.method == "POST":
+        username = request.form.get("username")
+        password = request.form.get("password")
+
+        if not username or not password:
+            flash("Please fill out all fields")
+            return render_template("register.html")
+
+        db = get_db()
+
+        existing_user = db.execute(
+            "SELECT id FROM users WHERE username = ?",
+            (username,)
+        ).fetchone()
+
+        if existing_user:
+            flash("Username already taken")
+            return render_template("register.html")
+
+        hashed_password = generate_password_hash(password)
+
+        db.execute(
+            "INSERT INTO users (username, password) VALUES (?, ?)",
+            (username, hashed_password)
+        )
+        db.commit()
+
+        flash("Account created! Please log in.")
+        return redirect(url_for("auth.login"))
+
+    return render_template("register.html")