fix: add vehicle button opens add vehicle form (refs #2)

- Add onAddVehicle prop to DashboardScreen
- Mobile: triggers setShowAddVehicle(true) in App.tsx
- Desktop: navigates to /garage/vehicles with showAddForm state
- VehiclesPage auto-opens form when receiving showAddForm state

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

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

frontend/src/features/dashboard/components/DashboardScreen.tsx

@@ -20,12 +20,14 @@ interface DashboardScreenProps {
   onNavigate?: (screen: MobileScreen, metadata?: Record<string, any>) => void;
   onVehicleClick?: (vehicle: Vehicle) => void;
   onViewMaintenance?: () => void;
+  onAddVehicle?: () => void;
 }
 
 export const DashboardScreen: React.FC<DashboardScreenProps> = ({
   onNavigate,
   onVehicleClick,
-  onViewMaintenance
+  onViewMaintenance,
+  onAddVehicle
 }) => {
   const { data: summary, isLoading: summaryLoading, error: summaryError } = useDashboardSummary();
   const { data: vehiclesNeedingAttention, isLoading: attentionLoading, error: attentionError } = useVehiclesNeedingAttention();
@@ -87,7 +89,7 @@ export const DashboardScreen: React.FC<DashboardScreenProps> = ({
             <Button
               variant="primary"
               size="lg"
-              onClick={() => onNavigate?.('Vehicles')}
+              onClick={onAddVehicle ?? (() => onNavigate?.('Vehicles'))}
             >
               Add Your First Vehicle
             </Button>
@@ -118,7 +120,7 @@ export const DashboardScreen: React.FC<DashboardScreenProps> = ({
 
       {/* Quick Actions */}
       <QuickActions
-        onAddVehicle={() => onNavigate?.('Vehicles')}
+        onAddVehicle={onAddVehicle ?? (() => onNavigate?.('Vehicles'))}
         onLogFuel={() => onNavigate?.('Log Fuel')}
         onViewMaintenance={onViewMaintenance ?? (() => onNavigate?.('Vehicles'))}
         onViewVehicles={() => onNavigate?.('Vehicles')}