feat: add dashboard with vehicle fleet overview (refs #2)

Implements responsive dashboard showing:
- Summary cards (vehicle count, upcoming maintenance, recent fuel logs)
- Vehicles needing attention with priority highlighting
- Quick action buttons for navigation
- Loading skeletons and empty states
- Mobile-first responsive layout (320px to 1920px+)

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

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

frontend/src/App.tsx

@@ -78,18 +78,7 @@ import { useDataSync } from './core/hooks/useDataSync';
 import { MobileDebugPanel } from './core/debug/MobileDebugPanel';
 import { MobileErrorBoundary } from './core/error-boundaries/MobileErrorBoundary';
 import { useLoginNotifications } from './features/notifications/hooks/useLoginNotifications';
-
-// Hoisted mobile screen components to stabilize identity and prevent remounts
-const DashboardScreen: React.FC = () => (
-  <div className="space-y-4">
-    <GlassCard>
-      <div className="text-center py-12">
-        <h2 className="text-lg font-semibold text-slate-800 mb-2">Dashboard</h2>
-        <p className="text-slate-500">Coming soon - Vehicle insights and analytics</p>
-      </div>
-    </GlassCard>
-  </div>
-);
+import { DashboardScreen as DashboardFeature } from './features/dashboard';
 
 const LogFuelScreen: React.FC = () => {
   const queryClient = useQueryClient();
@@ -640,7 +629,10 @@ function App() {
                 transition={{ duration: 0.2, ease: "easeOut" }}
               >
                 <MobileErrorBoundary screenName="Dashboard">
-                  <DashboardScreen />
+                  <DashboardFeature
+                    onNavigate={navigateToScreen}
+                    onVehicleClick={handleVehicleSelect}
+                  />
                 </MobileErrorBoundary>
               </motion.div>
             )}