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/core/store/navigation.ts

@@ -51,7 +51,7 @@ export const useNavigationStore = create<NavigationState>()(
   persist(
     (set, get) => ({
       // Initial state
-      activeScreen: 'Vehicles',
+      activeScreen: 'Dashboard',
       vehicleSubScreen: 'list',
       selectedVehicleId: null,
       navigationHistory: [],