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/vehicles/pages/VehiclesPage.tsx

@@ -3,7 +3,7 @@
  * @ai-context Enhanced with Suspense, useOptimistic, and useTransition
  */
 
-import React, { useState, useTransition, useMemo } from 'react';
+import React, { useState, useTransition, useMemo, useEffect } from 'react';
 import { Box, Typography, Grid, Button as MuiButton, TextField, IconButton } from '@mui/material';
 import AddIcon from '@mui/icons-material/Add';
 import SearchIcon from '@mui/icons-material/Search';
@@ -16,12 +16,13 @@ import { VehicleForm } from '../components/VehicleForm';
 import { Card } from '../../../shared-minimal/components/Card';
 import { VehicleListSuspense, FormSuspense } from '../../../components/SuspenseWrappers';
 import { useAppStore } from '../../../core/store';
-import { useNavigate } from 'react-router-dom';
+import { useNavigate, useLocation } from 'react-router-dom';
 import { useQueryClient } from '@tanstack/react-query';
 import { vehiclesApi } from '../api/vehicles.api';
 
 export const VehiclesPage: React.FC = () => {
   const navigate = useNavigate();
+  const location = useLocation();
   const queryClient = useQueryClient();
   const { data: vehicles, isLoading } = useVehicles();
   const setSelectedVehicle = useAppStore(state => state.setSelectedVehicle);
@@ -52,6 +53,16 @@ export const VehiclesPage: React.FC = () => {
   const [showForm, setShowForm] = useState(false);
   const [stagedImageFile, setStagedImageFile] = useState<File | null>(null);
 
+  // Auto-show form if navigated with showAddForm state (from dashboard)
+  useEffect(() => {
+    const state = location.state as { showAddForm?: boolean } | null;
+    if (state?.showAddForm) {
+      setShowForm(true);
+      // Clear the state to prevent re-triggering on refresh
+      navigate(location.pathname, { replace: true, state: {} });
+    }
+  }, [location.state, location.pathname, navigate]);
+
   const handleSelectVehicle = (id: string) => {
     // Use transition for navigation to avoid blocking UI
     startTransition(() => {