fix: add dashboard to navigation and set as default landing page (refs #2)

- Add Dashboard to desktop sidebar navigation (first item)
- Add /garage/dashboard route for desktop
- Change default redirect from /garage/vehicles to /garage/dashboard
- Change mobile default screen from Vehicles to Dashboard
- Create DashboardPage wrapper for desktop route

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/dashboard/index.ts

@@ -3,6 +3,7 @@
  */
 
 export { DashboardScreen } from './components/DashboardScreen';
+export { DashboardPage } from './pages/DashboardPage';
 export { SummaryCards, SummaryCardsSkeleton } from './components/SummaryCards';
 export { VehicleAttention, VehicleAttentionSkeleton } from './components/VehicleAttention';
 export { QuickActions, QuickActionsSkeleton } from './components/QuickActions';