refactor(frontend): Reorganize pages with tabbed Settings and focused System page

- Create new tabbed Settings component combining accounts and notifications
- Extract sync operations into dedicated System component
- Update routing: /notifications → /system with proper navigation labels
- Remove duplicate page headers (using existing SiteHeader)
- Add shadcn tabs component for better UX
- Fix mypy error in database_service.py (handle None lastrowid)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/routes/system.tsx

@@ -0,0 +1,6 @@
+import { createFileRoute } from "@tanstack/react-router";
+import System from "../components/System";
+
+export const Route = createFileRoute("/system")({
+  component: System,
+});