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/App.tsx

@@ -633,6 +633,7 @@ function App() {
                   <DashboardFeature
                     onNavigate={navigateToScreen}
                     onVehicleClick={handleVehicleSelect}
+                    onAddVehicle={() => setShowAddVehicle(true)}
                   />
                 </MobileErrorBoundary>
               </motion.div>