fix: extract collection dialogs into own StatefulWidgets

Same _dependents.isEmpty crash as the password dialog  the Add/Already
Exists dialogs were sharing the parent context across async DB calls.
Now each dialog is an independent widget with its own lifecycle.

lib/main.dart

@@ -380,15 +380,12 @@ class HomeScreen extends StatelessWidget {
 
   /// Opens the scanner, gets the hw_id, then queries the DB.
   Future<void> _openScanner(BuildContext context) async {
-    // 1. Navigate to the scanner screen and wait for the result.
     final hwId = await Navigator.of(context).push<String>(
       MaterialPageRoute(builder: (_) => const ScannerScreen()),
     );
 
-    // User cancelled / went back without selecting an ID.
     if (hwId == null || !context.mounted) return;
 
-    // 2. Query the database.
     try {
       final data = await supabase
           .from('hotwheels')
@@ -399,11 +396,15 @@ class HomeScreen extends StatelessWidget {
       if (!context.mounted) return;
 
       if (data != null) {
-        // ── Car already exists ──
-        _showAlreadyExistsDialog(context, hwId);
+        showDialog<void>(
+          context: context,
+          builder: (_) => _AlreadyExistsDialog(hwId: hwId),
+        );
       } else {
-        // ── Car is new — offer to add it ──
-        _showAddDialog(context, hwId);
+        showDialog<void>(
+          context: context,
+          builder: (_) => _AddCarDialog(hwId: hwId),
+        );
       }
     } catch (e) {
       if (!context.mounted) return;
@@ -412,62 +413,85 @@ class HomeScreen extends StatelessWidget {
       );
     }
   }
+}
 
-  /// Shows an alert: this car is already in the collection.
-  void _showAlreadyExistsDialog(BuildContext context, String hwId) {
-    showDialog<void>(
-      context: context,
-      builder: (ctx) => AlertDialog(
+// ── "Already in Collection" Dialog ────────────────────────────────────
+class _AlreadyExistsDialog extends StatelessWidget {
+  final String hwId;
+  const _AlreadyExistsDialog({required this.hwId});
+
+  @override
+  Widget build(BuildContext context) {
+    return AlertDialog(
       icon: const Icon(Icons.check_circle, color: Colors.green, size: 48),
       title: const Text('Already in Collection!'),
       content: Text('$hwId is already in your shared garage.'),
       actions: [
         ElevatedButton(
-            onPressed: () => Navigator.of(ctx).pop(),
+          onPressed: () => Navigator.of(context).pop(),
           child: const Text('Got it'),
         ),
       ],
-      ),
     );
   }
+}
+
+// ── "Add to Collection" Dialog ────────────────────────────────────────
+class _AddCarDialog extends StatefulWidget {
+  final String hwId;
+  const _AddCarDialog({required this.hwId});
+
+  @override
+  State<_AddCarDialog> createState() => _AddCarDialogState();
+}
+
+class _AddCarDialogState extends State<_AddCarDialog> {
+  bool _isAdding = false;
+
+  Future<void> _addCar() async {
+    setState(() => _isAdding = true);
 
-  /// Shows a confirmation dialog to add a new car.
-  void _showAddDialog(BuildContext context, String hwId) {
-    showDialog<void>(
-      context: context,
-      builder: (ctx) => AlertDialog(
-        icon: const Icon(Icons.add_circle_outline, color: Colors.deepPurple, size: 48),
-        title: const Text('New Car Found!'),
-        content: Text('$hwId is not in your collection yet.\nAdd it now?'),
-        actions: [
-          TextButton(
-            onPressed: () => Navigator.of(ctx).pop(),
-            child: const Text('Cancel'),
-          ),
-          ElevatedButton(
-            onPressed: () async {
     try {
       await supabase.from('hotwheels').insert({
-                  'hw_id': hwId,
+        'hw_id': widget.hwId,
         'user_id': supabase.auth.currentUser!.id,
       });
-                if (!ctx.mounted) return;
-                Navigator.of(ctx).pop();
+      if (!mounted) return;
+      Navigator.of(context).pop();
       ScaffoldMessenger.of(context).showSnackBar(
-                  SnackBar(content: Text('$hwId added to your collection! 🎉')),
+        SnackBar(content: Text('${widget.hwId} added to your collection! 🎉')),
       );
     } catch (e) {
-                if (!ctx.mounted) return;
-                Navigator.of(ctx).pop();
+      if (!mounted) return;
+      setState(() => _isAdding = false);
       ScaffoldMessenger.of(context).showSnackBar(
         SnackBar(content: Text('Failed to add: $e'), backgroundColor: Colors.red),
       );
     }
-            },
-            child: const Text('Add to Collection'),
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return AlertDialog(
+      icon: const Icon(Icons.add_circle_outline, color: Colors.deepPurple, size: 48),
+      title: const Text('New Car Found!'),
+      content: Text('${widget.hwId} is not in your collection yet.\nAdd it now?'),
+      actions: [
+        TextButton(
+          onPressed: _isAdding ? null : () => Navigator.of(context).pop(),
+          child: const Text('Cancel'),
+        ),
+        ElevatedButton(
+          onPressed: _isAdding ? null : _addCar,
+          child: _isAdding
+              ? const SizedBox(
+                  height: 18,
+                  width: 18,
+                  child: CircularProgressIndicator(strokeWidth: 2),
+                )
+              : const Text('Add to Collection'),
         ),
       ],
-      ),
     );
   }
 }