Add category management UI and service layer

Users can now create, edit, and delete custom categories from Settings → Categories. System categories (45 built-in) are shown read-only. Backend adds update_category() and delete_category() service functions; frontend has a new categories API module and a full CRUD section in SettingsPage with filter tabs, colour picker, and delete confirmation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/category_service.py

@@ -133,3 +133,51 @@ async def create_category(
     db.add(cat)
     await db.flush()
     return {"id": str(cat.id), "name": cat.name, "type": cat.type, "icon": cat.icon, "color": cat.color, "is_system": False}
+
+
+async def update_category(
+    db: AsyncSession,
+    user_id: uuid.UUID,
+    category_id: uuid.UUID,
+    name: str | None,
+    color: str | None,
+    icon: str | None,
+) -> dict | None:
+    result = await db.execute(
+        select(Category).where(
+            Category.id == category_id,
+            Category.user_id == user_id,
+            Category.is_system == False,
+        )
+    )
+    cat = result.scalar_one_or_none()
+    if cat is None:
+        return None
+    if name is not None:
+        cat.name = name
+    if color is not None:
+        cat.color = color
+    if icon is not None:
+        cat.icon = icon
+    await db.flush()
+    return {"id": str(cat.id), "name": cat.name, "type": cat.type, "icon": cat.icon, "color": cat.color, "is_system": False}
+
+
+async def delete_category(
+    db: AsyncSession,
+    user_id: uuid.UUID,
+    category_id: uuid.UUID,
+) -> bool:
+    result = await db.execute(
+        select(Category).where(
+            Category.id == category_id,
+            Category.user_id == user_id,
+            Category.is_system == False,
+        )
+    )
+    cat = result.scalar_one_or_none()
+    if cat is None:
+        return False
+    await db.delete(cat)
+    await db.flush()
+    return True