Add theming system: custom palettes, per-user colour mode, admin appearance page

- 4 built-in themes (Default, Pastel, High Contrast, Ocean Blue) seeded as
  JSON files in /config/themes/ on startup; custom themes can be created,
  edited, and deleted via the new admin Appearance page
- All theme tokens applied via JS inline CSS properties (no hardcoded CSS blocks)
- New `color_mode` column on users table (migration dd6ad2f2c211); users can
  override the admin-set global default in Settings
- Backend: GET/PATCH /settings/appearance, full CRUD on /settings/themes
- Frontend: AdminAppearancePage with theme grid + colour pickers, SettingsPage
  replaces placeholder with mode selector, useTheme rewritten to fetch from API

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

backend/app/routers/users.py

@@ -1,10 +1,10 @@
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
 from app.deps import get_current_user
 from app.models.user import User
-from app.schemas.user import DashboardPrefsOut, DashboardPrefsUpdate, UserOut
+from app.schemas.user import ColorModeUpdate, DashboardPrefsOut, DashboardPrefsUpdate, UserOut
 
 router = APIRouter()
 
@@ -29,3 +29,15 @@ async def update_preferences(
     await db.commit()
     await db.refresh(current_user)
     return DashboardPrefsOut(app_ids=current_user.dashboard_app_ids or [])
+
+
+@router.patch("/me/color-mode", response_model=UserOut)
+async def update_color_mode(
+    body: ColorModeUpdate,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    current_user.color_mode = body.color_mode
+    await db.commit()
+    await db.refresh(current_user)
+    return current_user