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/main.py

@@ -4,6 +4,7 @@ from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
+from app.core.app_config import seed_builtin_themes
 from app.core.config import settings
 from app.routers import admin, auth, categories_proxy, documents_proxy, groups, profile, services, users
 from app.routers import settings as settings_router
@@ -12,6 +13,7 @@ from app.services.service_health import check_all, health_check_loop, register_s
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    await asyncio.to_thread(seed_builtin_themes)
     register_services(
         doc_service_url=settings.DOC_SERVICE_URL,
         ai_service_url=settings.AI_SERVICE_URL,