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/AppSidebar.tsx

@@ -33,7 +33,7 @@ import {
 const navigation = [
   { name: "Overview", icon: List, to: "/" },
   { name: "Analytics", icon: BarChart3, to: "/analytics" },
-  { name: "System Status", icon: Activity, to: "/notifications" },
+  { name: "System", icon: Activity, to: "/system" },
   { name: "Settings", icon: Settings, to: "/settings" },
 ];