fix(state): scope active collection preference by user with legacy key migration

lib/main.dart

@@ -7,6 +7,7 @@ import 'services/main_collection_sync.dart';
 import 'screens/login_screen.dart';
 import 'screens/home_shell.dart';
 import 'utils/error_utils.dart';
+import 'utils/preferences_utils.dart';
 
 // Re-export so other files can `import '../main.dart'` for these.
 export 'package:supabase_flutter/supabase_flutter.dart'
@@ -118,8 +119,6 @@ class AuthGate extends StatefulWidget {
 }
 
 class _AuthGateState extends State<AuthGate> {
-  static const _activeCollectionPrefKey = 'active_collection_id';
-
   bool _isLoading = true;
   bool _isInPasswordRecoveryFlow = false;
   Session? _session;
@@ -183,7 +182,7 @@ class _AuthGateState extends State<AuthGate> {
     if (userId == null) return;
 
     final prefs = await SharedPreferences.getInstance();
-    final persisted = prefs.getString(_activeCollectionPrefKey);
+    final persisted = await readActiveCollectionId(prefs, userId: userId);
 
     Future<bool> hasMembership(String collectionId) async {
       final membership = await supabase
@@ -214,11 +213,15 @@ class _AuthGateState extends State<AuthGate> {
     }
 
     if (nextActiveId == null) {
-      await prefs.remove(_activeCollectionPrefKey);
+      await clearActiveCollectionId(prefs, userId: userId);
       return;
     }
 
-    await prefs.setString(_activeCollectionPrefKey, nextActiveId);
+    await writeActiveCollectionId(
+      prefs,
+      userId: userId,
+      collectionId: nextActiveId,
+    );
     MainCollectionSync.notifyChanged();
   }
 

lib/screens/collections_screen.dart

@@ -5,6 +5,7 @@ import '../services/collection_service.dart';
 import '../services/main_collection_sync.dart';
 import '../theme/app_colors.dart';
 import '../utils/error_utils.dart';
+import '../utils/preferences_utils.dart';
 import 'garage_screen.dart';
 import 'manage_collection_screen.dart';
 
@@ -18,8 +19,6 @@ class CollectionsScreen extends StatefulWidget {
 
 class CollectionsScreenState extends State<CollectionsScreen>
   with WidgetsBindingObserver {
-  static const _activeCollectionPrefKey = 'active_collection_id';
-
   List<Collection> _collections = [];
   bool _isLoading = true;
   String? _error;
@@ -74,8 +73,13 @@ class CollectionsScreenState extends State<CollectionsScreen>
         await CollectionService.ensureDefaultCollection();
         list = await CollectionService.getMyCollections();
       }
+      final userId = supabase.auth.currentUser?.id;
+      if (userId == null) {
+        throw Exception('You must be signed in to load collections.');
+      }
+
       final prefs = await SharedPreferences.getInstance();
-      final persisted = prefs.getString(_activeCollectionPrefKey);
+      final persisted = await readActiveCollectionId(prefs, userId: userId);
 
       String? activeId = persisted;
       if (activeId == null && list.isNotEmpty) {
@@ -96,7 +100,11 @@ class CollectionsScreenState extends State<CollectionsScreen>
       });
 
       if (activeId != null) {
-        await prefs.setString(_activeCollectionPrefKey, activeId);
+        await writeActiveCollectionId(
+          prefs,
+          userId: userId,
+          collectionId: activeId,
+        );
         if (shouldNotifySync) {
           MainCollectionSync.notifyChanged();
         }
@@ -218,8 +226,18 @@ class CollectionsScreenState extends State<CollectionsScreen>
   }
 
   Future<void> _setActiveCollection(String collectionId) async {
+    final userId = supabase.auth.currentUser?.id;
+    if (userId == null) {
+      showGlobalSnackBar('Please sign in again.', isError: true);
+      return;
+    }
+
     final prefs = await SharedPreferences.getInstance();
-    await prefs.setString(_activeCollectionPrefKey, collectionId);
+    await writeActiveCollectionId(
+      prefs,
+      userId: userId,
+      collectionId: collectionId,
+    );
     if (!mounted) return;
     setState(() => _activeCollectionId = collectionId);
     MainCollectionSync.notifyChanged();

lib/screens/scan_tab.dart

@@ -5,6 +5,7 @@ import '../scanner_screen.dart';
 import '../services/collection_service.dart';
 import '../services/main_collection_sync.dart';
 import '../theme/app_colors.dart';
+import '../utils/preferences_utils.dart';
 
 class ScanTab extends StatefulWidget {
   const ScanTab({super.key});
@@ -14,7 +15,6 @@ class ScanTab extends StatefulWidget {
 }
 
 class ScanTabState extends State<ScanTab> {
-  static const _activeCollectionPrefKey = 'active_collection_id';
   static const _duplicateCooldown = Duration(seconds: 2);
 
   bool _isBusy = false;
@@ -50,8 +50,13 @@ class ScanTabState extends State<ScanTab> {
   Future<void> _loadCollections() async {
     try {
       final list = await CollectionService.getMyCollections();
+      final userId = supabase.auth.currentUser?.id;
+      if (userId == null) {
+        throw Exception('You must be signed in to load collections.');
+      }
+
       final prefs = await SharedPreferences.getInstance();
-      final persistedId = prefs.getString(_activeCollectionPrefKey);
+      final persistedId = await readActiveCollectionId(prefs, userId: userId);
 
       Collection? selected;
       if (persistedId != null) {
@@ -71,7 +76,11 @@ class ScanTabState extends State<ScanTab> {
       });
 
       if (selected != null) {
-        await prefs.setString(_activeCollectionPrefKey, selected.id);
+        await writeActiveCollectionId(
+          prefs,
+          userId: userId,
+          collectionId: selected.id,
+        );
       }
     } catch (e) {
       if (!mounted) return;

lib/utils/preferences_utils.dart

@@ -0,0 +1,43 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+const _legacyActiveCollectionPrefKey = 'active_collection_id';
+
+String activeCollectionPrefKeyForUser(String userId) {
+  return 'active_collection_id_$userId';
+}
+
+Future<String?> readActiveCollectionId(
+  SharedPreferences prefs, {
+  required String userId,
+}) async {
+  final scopedKey = activeCollectionPrefKeyForUser(userId);
+  final scopedValue = prefs.getString(scopedKey);
+  if (scopedValue != null && scopedValue.isNotEmpty) {
+    return scopedValue;
+  }
+
+  final legacyValue = prefs.getString(_legacyActiveCollectionPrefKey);
+  if (legacyValue == null || legacyValue.isEmpty) {
+    return null;
+  }
+
+  await prefs.setString(scopedKey, legacyValue);
+  await prefs.remove(_legacyActiveCollectionPrefKey);
+  return legacyValue;
+}
+
+Future<void> writeActiveCollectionId(
+  SharedPreferences prefs, {
+  required String userId,
+  required String collectionId,
+}) {
+  return prefs.setString(activeCollectionPrefKeyForUser(userId), collectionId);
+}
+
+Future<void> clearActiveCollectionId(
+  SharedPreferences prefs, {
+  required String userId,
+}) async {
+  await prefs.remove(activeCollectionPrefKeyForUser(userId));
+  await prefs.remove(_legacyActiveCollectionPrefKey);
+}