fix(ux): show errors above modals and handle duplicate owner moves cleanly

lib/main.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'package:supabase_flutter/supabase_flutter.dart';
@@ -49,9 +51,16 @@ final supabase = Supabase.instance.client;
 /// Global keys so dialogs & snackbars survive widget-tree rebuilds.
 final navigatorKey = GlobalKey<NavigatorState>();
 final scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
+OverlayEntry? _activeErrorOverlay;
+Timer? _activeErrorOverlayTimer;
 
 /// Show a snackbar safely through the global key.
 void showGlobalSnackBar(String message, {bool isError = false}) {
+  if (isError) {
+    _showGlobalErrorOverlay(message);
+    return;
+  }
+
   final messenger = scaffoldMessengerKey.currentState;
   if (messenger == null) return;
 
@@ -68,6 +77,67 @@ void showGlobalSnackBar(String message, {bool isError = false}) {
   );
 }
 
+void _showGlobalErrorOverlay(String message) {
+  final overlay = navigatorKey.currentState?.overlay;
+  if (overlay == null) return;
+
+  _activeErrorOverlayTimer?.cancel();
+  _activeErrorOverlay?.remove();
+
+  _activeErrorOverlay = OverlayEntry(
+    builder: (context) {
+      final topPadding = MediaQuery.of(context).padding.top;
+      return Positioned(
+        top: topPadding + 12,
+        left: 12,
+        right: 12,
+        child: Material(
+          color: Colors.transparent,
+          child: IgnorePointer(
+            ignoring: true,
+            child: Container(
+              padding: const EdgeInsets.symmetric(horizontal: 14, vertical: 12),
+              decoration: BoxDecoration(
+                color: Colors.red,
+                borderRadius: BorderRadius.circular(12),
+                boxShadow: const [
+                  BoxShadow(
+                    color: Colors.black26,
+                    blurRadius: 8,
+                    offset: Offset(0, 3),
+                  ),
+                ],
+              ),
+              child: Row(
+                children: [
+                  const Icon(Icons.error_outline, color: Colors.white, size: 20),
+                  const SizedBox(width: 8),
+                  Expanded(
+                    child: Text(
+                      message,
+                      style: const TextStyle(color: Colors.white),
+                      maxLines: 3,
+                      overflow: TextOverflow.ellipsis,
+                    ),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+    },
+  );
+
+  overlay.insert(_activeErrorOverlay!);
+
+  _activeErrorOverlayTimer = Timer(const Duration(seconds: 4), () {
+    _activeErrorOverlay?.remove();
+    _activeErrorOverlay = null;
+    _activeErrorOverlayTimer = null;
+  });
+}
+
 void showGlobalError(
   Object error, {
   String fallback = 'Something went wrong. Please try again.',

lib/screens/garage_screen.dart

@@ -479,10 +479,49 @@ class GarageScreenState extends State<GarageScreen> {
       final targetCollectionId = targetId!;
 
       if (isOwner) {
+        final sourceCars = _cars
+            .where((car) => _selectedIds.contains(car['id'] as int))
+            .toList(growable: false);
+
+        final hwIds = sourceCars
+            .map((car) => car['hw_id'] as String)
+            .toSet()
+            .toList(growable: false);
+
+        final existing = await supabase
+            .from('hotwheels')
+            .select('hw_id')
+            .eq('collection_id', targetCollectionId)
+            .inFilter('hw_id', hwIds);
+
+        final existingHwIds = (existing as List)
+            .map((row) => row['hw_id'] as String)
+            .toSet();
+
+        final moveableIds = sourceCars
+            .where((car) => !existingHwIds.contains(car['hw_id'] as String))
+            .map((car) => car['id'] as int)
+            .toList(growable: false);
+
+        final skippedDuplicates = sourceCars.length - moveableIds.length;
+        if (moveableIds.isEmpty) {
+          showGlobalSnackBar(
+            'All selected cars are already in the target collection.',
+            isError: true,
+          );
+          return;
+        }
+
         await supabase
             .from('hotwheels')
-        .update({'collection_id': targetCollectionId})
+            .update({'collection_id': targetCollectionId})
-            .inFilter('id', _selectedIds.toList());
+            .inFilter('id', moveableIds);
+
+        if (skippedDuplicates > 0) {
+          showGlobalSnackBar(
+            '$skippedDuplicates car(s) skipped because they already exist in target collection.',
+          );
+        }
       } else {
         final userId = supabase.auth.currentUser?.id;
         if (userId == null) {
@@ -1092,6 +1131,21 @@ class GarageScreenState extends State<GarageScreen> {
       if (targetId == null) return;
 
       if (widget.isOwner) {
+        final existing = await supabase
+            .from('hotwheels')
+            .select('id')
+            .eq('collection_id', targetId)
+            .eq('hw_id', car['hw_id'])
+            .maybeSingle();
+
+        if (existing != null) {
+          showGlobalSnackBar(
+            '${car['hw_id']} is already in the target collection.',
+            isError: true,
+          );
+          return;
+        }
+
         await supabase
             .from('hotwheels')
             .update({'collection_id': targetId})