fix: use global navigator/scaffold keys for all dialogs and snackbars

Eliminates all _dependents.isEmpty and Duplicate GlobalKeys crashes by
routing every dialog through a global NavigatorState key and every
snackbar through a global ScaffoldMessengerState key. These keys live
on MaterialApp and are never disposed, so async operations can safely
show UI feedback regardless of widget tree rebuilds from auth events.

lib/main.dart

@@ -20,6 +20,28 @@ Future<void> main() async {
 /// Convenience accessor used throughout the app.
 final supabase = Supabase.instance.client;
 
+/// Global keys so dialogs & snackbars survive widget-tree rebuilds.
+final navigatorKey = GlobalKey<NavigatorState>();
+final scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
+
+/// Show a snackbar safely through the global key.
+void showGlobalSnackBar(String message, {bool isError = false}) {
+  scaffoldMessengerKey.currentState?.showSnackBar(
+    SnackBar(
+      content: Text(message),
+      backgroundColor: isError ? Colors.red : null,
+    ),
+  );
+}
+
+/// Show a dialog safely through the global navigator key.
+Future<T?> showGlobalDialog<T>({required WidgetBuilder builder}) {
+  return showDialog<T>(
+    context: navigatorKey.currentContext!,
+    builder: builder,
+  );
+}
+
 // ── Root App Widget ───────────────────────────────────────────────────
 class HWHubApp extends StatelessWidget {
   const HWHubApp({super.key});
@@ -29,6 +51,8 @@ class HWHubApp extends StatelessWidget {
     return MaterialApp(
       title: 'HW Hub',
       debugShowCheckedModeBanner: false,
+      navigatorKey: navigatorKey,
+      scaffoldMessengerKey: scaffoldMessengerKey,
       theme: ThemeData(
         colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
         useMaterial3: true,
@@ -82,10 +106,7 @@ class _AuthGateState extends State<AuthGate> {
         }
       },
       onError: (error) {
-        if (!mounted) return;
-        ScaffoldMessenger.of(context).showSnackBar(
-          SnackBar(content: Text('Auth error: $error'), backgroundColor: Colors.red),
-        );
+        showGlobalSnackBar('Auth error: $error', isError: true);
       },
     );
 
@@ -96,10 +117,8 @@ class _AuthGateState extends State<AuthGate> {
   /// Shows a dialog so the user can type a new password after clicking
   /// the "Reset Password" link from their email.
   Future<void> _showResetPasswordDialog() async {
-    if (!mounted) return;
-
     await showDialog<void>(
-      context: context,
+      context: navigatorKey.currentContext!,
       barrierDismissible: false,
       builder: (_) => const _ResetPasswordDialog(),
     );
@@ -142,9 +161,7 @@ class _ResetPasswordDialogState extends State<_ResetPasswordDialog> {
   Future<void> _save() async {
     final newPassword = _passwordController.text.trim();
     if (newPassword.length < 6) {
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('Password must be at least 6 characters.')),
-      );
+      showGlobalSnackBar('Password must be at least 6 characters.');
       return;
     }
 
@@ -156,15 +173,11 @@ class _ResetPasswordDialogState extends State<_ResetPasswordDialog> {
       );
       if (!mounted) return;
       Navigator.of(context).pop();
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('Password updated successfully!')),
-      );
+      showGlobalSnackBar('Password updated successfully!');
     } on AuthException catch (e) {
       if (!mounted) return;
       setState(() => _isSaving = false);
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text(e.message), backgroundColor: Colors.red),
-      );
+      showGlobalSnackBar(e.message, isError: true);
     }
   }
 
@@ -230,9 +243,7 @@ class _LoginScreenState extends State<LoginScreen> {
       // AuthGate listener will pick up the new session automatically.
     } on AuthException catch (e) {
       if (!mounted) return;
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text(e.message), backgroundColor: Colors.red),
-      );
+      showGlobalSnackBar(e.message, isError: true);
     } finally {
       if (mounted) setState(() => _isLoading = false);
     }
@@ -244,9 +255,7 @@ class _LoginScreenState extends State<LoginScreen> {
   Future<void> _forgotPassword() async {
     final email = _emailController.text.trim();
     if (email.isEmpty) {
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('Enter your email first.')),
-      );
+      showGlobalSnackBar('Enter your email first.');
       return;
     }
 
@@ -255,15 +264,9 @@ class _LoginScreenState extends State<LoginScreen> {
         email,
         redirectTo: 'hwcollector://login',
       );
-      if (!mounted) return;
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('Password reset email sent! Check your inbox.')),
-      );
+      showGlobalSnackBar('Password reset email sent! Check your inbox.');
     } on AuthException catch (e) {
-      if (!mounted) return;
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text(e.message), backgroundColor: Colors.red),
-      );
+      showGlobalSnackBar(e.message, isError: true);
     }
   }
 
@@ -393,11 +396,11 @@ class _HomeScreenState extends State<HomeScreen> {
 
   /// Opens the scanner, gets the hw_id, then queries the DB.
   Future<void> _openScanner() async {
-    final hwId = await Navigator.of(context).push<String>(
+    final hwId = await navigatorKey.currentState!.push<String>(
       MaterialPageRoute(builder: (_) => const ScannerScreen()),
     );
 
-    if (hwId == null || !mounted) return;
+    if (hwId == null) return;
 
     setState(() => _isBusy = true);
 
@@ -408,32 +411,23 @@ class _HomeScreenState extends State<HomeScreen> {
           .eq('hw_id', hwId)
           .maybeSingle();
 
-      if (!mounted) return;
-      setState(() => _isBusy = false);
+      if (mounted) setState(() => _isBusy = false);
 
       if (data != null) {
-        await showDialog<void>(
-          context: context,
+        await showGlobalDialog(
           builder: (_) => _AlreadyExistsDialog(hwId: hwId),
         );
       } else {
-        // Dialog returns true if the car was added successfully.
-        final added = await showDialog<bool>(
-          context: context,
+        final added = await showGlobalDialog<bool>(
           builder: (_) => _AddCarDialog(hwId: hwId),
         );
-        if (added == true && mounted) {
-          ScaffoldMessenger.of(context).showSnackBar(
-            SnackBar(content: Text('$hwId added to your collection! 🎉')),
-          );
+        if (added == true) {
+          showGlobalSnackBar('$hwId added to your collection! 🎉');
         }
       }
     } catch (e) {
-      if (!mounted) return;
-      setState(() => _isBusy = false);
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text('DB error: $e'), backgroundColor: Colors.red),
-      );
+      if (mounted) setState(() => _isBusy = false);
+      showGlobalSnackBar('DB error: $e', isError: true);
     }
   }
 }
@@ -480,14 +474,11 @@ class _AddCarDialogState extends State<_AddCarDialog> {
         'user_id': supabase.auth.currentUser!.id,
       });
       if (!mounted) return;
-      // Return true to signal success — snackbar shown by HomeScreen.
       Navigator.of(context).pop(true);
     } catch (e) {
       if (!mounted) return;
       setState(() => _isAdding = false);
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text('Failed to add: $e'), backgroundColor: Colors.red),
-      );
+      showGlobalSnackBar('Failed to add: $e', isError: true);
     }
   }