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/components/SiteHeader.tsx

@@ -10,7 +10,7 @@ const navigation = [
   { name: "Overview", to: "/" },
   { name: "Transactions", to: "/transactions" },
   { name: "Analytics", to: "/analytics" },
-  { name: "Notifications", to: "/notifications" },
+  { name: "System", to: "/system" },
   { name: "Settings", to: "/settings" },
 ];