feat(collections): persist active collection and harden member actions

- Persist scanner activeCollectionId via shared_preferences
- Restore last selected collection automatically on scan tab load
- Keep selection valid if collection list changes and store fallback safely
- Harden collection membership service rules:
  - only owners can invite/remove members
  - prevent self-invite duplicates
  - block owner removal
  - block owner leave action with explicit error
- Improve manage-collection UX with pull-to-refresh and owner leave guidance

lib/screens/manage_collection_screen.dart

@@ -321,128 +321,136 @@ class _ManageCollectionScreenState extends State<ManageCollectionScreen> {
             ),
         ],
       ),
-      body: ListView(
+      body: RefreshIndicator(
-        padding: const EdgeInsets.all(16),
+        onRefresh: _loadMembers,
-        children: [
+        child: ListView(
-          // ── Description ──
+          padding: const EdgeInsets.all(16),
-          if (_collection.description != null &&
+          children: [
-              _collection.description!.isNotEmpty) ...[
+            if (_collection.description != null &&
-            Text(
+                _collection.description!.isNotEmpty) ...[
-              _collection.description!,
-              style: const TextStyle(
-                  fontSize: 14, color: AppColors.textSecondary),
-            ),
-            const SizedBox(height: 16),
-          ],
-
-          // ── Members section ──
-          Row(
-            children: [
               Text(
-                'Members',
+                _collection.description!,
-                style: theme.textTheme.titleMedium?.copyWith(
+                style: const TextStyle(
-                  fontWeight: FontWeight.w600,
+                    fontSize: 14, color: AppColors.textSecondary),
-                ),
               ),
-              const Spacer(),
+              const SizedBox(height: 16),
-              if (_collection.isOwner)
-                TextButton.icon(
-                  onPressed: _inviteMember,
-                  icon: const Icon(Icons.person_add, size: 18),
-                  label: const Text('Invite'),
-                ),
             ],
-          ),
-          const SizedBox(height: 8),
 
-          if (_isLoading)
+            Row(
-            const Center(
+              children: [
-              child: Padding(
+                Text(
-                padding: EdgeInsets.all(24),
+                  'Members',
-                child: CircularProgressIndicator(),
+                  style: theme.textTheme.titleMedium?.copyWith(
-              ),
+                    fontWeight: FontWeight.w600,
-            )
+                  ),
-          else
+                ),
-            ...List.generate(_members.length, (i) {
+                const Spacer(),
-              final m = _members[i];
+                if (_collection.isOwner)
-              return Card(
+                  TextButton.icon(
-                margin: const EdgeInsets.only(bottom: 8),
+                    onPressed: _inviteMember,
-                child: ListTile(
+                    icon: const Icon(Icons.person_add, size: 18),
-                  leading: CircleAvatar(
+                    label: const Text('Invite'),
-                    backgroundColor: m.isOwner
+                  ),
-                        ? AppColors.orange
+              ],
-                        : AppColors.navy,
+            ),
-                    child: Icon(
+            const SizedBox(height: 8),
-                      m.isOwner ? Icons.star : Icons.person,
+
-                      color: Colors.white,
+            if (_isLoading)
-                      size: 20,
+              const Center(
+                child: Padding(
+                  padding: EdgeInsets.all(24),
+                  child: CircularProgressIndicator(),
+                ),
+              )
+            else
+              ...List.generate(_members.length, (i) {
+                final member = _members[i];
+                return Card(
+                  margin: const EdgeInsets.only(bottom: 8),
+                  child: ListTile(
+                    leading: CircleAvatar(
+                      backgroundColor:
+                          member.isOwner ? AppColors.orange : AppColors.navy,
+                      child: Icon(
+                        member.isOwner ? Icons.star : Icons.person,
+                        color: Colors.white,
+                        size: 20,
+                      ),
                     ),
+                    title: Text(
+                      member.email,
+                      style: const TextStyle(fontWeight: FontWeight.w500),
+                    ),
+                    subtitle: Text(
+                      member.isOwner ? 'Owner' : 'Member',
+                      style: const TextStyle(fontSize: 12),
+                    ),
+                    trailing: (!member.isOwner &&
+                            _collection.isOwner &&
+                            member.userId != currentUserId)
+                        ? IconButton(
+                            icon: const Icon(Icons.remove_circle_outline,
+                                color: AppColors.error),
+                            onPressed: () => _removeMember(member),
+                          )
+                        : null,
                   ),
-                  title: Text(
+                );
-                    m.email,
+              }),
-                    style: const TextStyle(fontWeight: FontWeight.w500),
-                  ),
-                  subtitle: Text(
-                    m.isOwner ? 'Owner' : 'Member',
-                    style: const TextStyle(fontSize: 12),
-                  ),
-                  trailing: (!m.isOwner &&
-                          _collection.isOwner &&
-                          m.userId != currentUserId)
-                      ? IconButton(
-                          icon: const Icon(Icons.remove_circle_outline,
-                              color: AppColors.error),
-                          onPressed: () => _removeMember(m),
-                        )
-                      : null,
-                ),
-              );
-            }),
 
-          const SizedBox(height: 32),
+            const SizedBox(height: 32),
-          const Divider(),
+            const Divider(),
-          const SizedBox(height: 16),
+            const SizedBox(height: 16),
 
-          // ── Danger zone ──
+            Text(
-          Text(
+              'Danger Zone',
-            'Danger Zone',
+              style: theme.textTheme.titleMedium?.copyWith(
-            style: theme.textTheme.titleMedium?.copyWith(
+                fontWeight: FontWeight.w600,
-              fontWeight: FontWeight.w600,
+                color: AppColors.error,
-              color: AppColors.error,
-            ),
-          ),
-          const SizedBox(height: 12),
-
-          if (!_collection.isOwner)
-            SizedBox(
-              width: double.infinity,
-              child: OutlinedButton.icon(
-                onPressed: _leaveCollection,
-                icon: const Icon(Icons.exit_to_app, color: AppColors.error),
-                label: const Text('Leave Collection',
-                    style: TextStyle(color: AppColors.error)),
-                style: OutlinedButton.styleFrom(
-                  side: const BorderSide(color: AppColors.error),
-                  padding: const EdgeInsets.symmetric(vertical: 14),
-                ),
               ),
             ),
+            const SizedBox(height: 12),
 
-          if (_collection.isOwner)
+            if (!_collection.isOwner)
-            SizedBox(
+              SizedBox(
-              width: double.infinity,
+                width: double.infinity,
-              child: OutlinedButton.icon(
+                child: OutlinedButton.icon(
-                onPressed: _deleteCollection,
+                  onPressed: _leaveCollection,
-                icon: const Icon(Icons.delete_forever, color: AppColors.error),
+                  icon: const Icon(Icons.exit_to_app, color: AppColors.error),
-                label: const Text('Delete Collection',
+                  label: const Text('Leave Collection',
-                    style: TextStyle(color: AppColors.error)),
+                      style: TextStyle(color: AppColors.error)),
-                style: OutlinedButton.styleFrom(
+                  style: OutlinedButton.styleFrom(
-                  side: const BorderSide(color: AppColors.error),
+                    side: const BorderSide(color: AppColors.error),
-                  padding: const EdgeInsets.symmetric(vertical: 14),
+                    padding: const EdgeInsets.symmetric(vertical: 14),
+                  ),
+                ),
+              )
+            else ...[
+              const Text(
+                'As owner, you cannot leave this collection. You can delete it instead.',
+                style: TextStyle(
+                  fontSize: 12,
+                  color: AppColors.textSecondary,
                 ),
               ),
-            ),
+              const SizedBox(height: 10),
-        ],
+              SizedBox(
+                width: double.infinity,
+                child: OutlinedButton.icon(
+                  onPressed: _deleteCollection,
+                  icon: const Icon(Icons.delete_forever,
+                      color: AppColors.error),
+                  label: const Text('Delete Collection',
+                      style: TextStyle(color: AppColors.error)),
+                  style: OutlinedButton.styleFrom(
+                    side: const BorderSide(color: AppColors.error),
+                    padding: const EdgeInsets.symmetric(vertical: 14),
+                  ),
+                ),
+              ),
+            ],
+          ],
+        ),
       ),
     );
   }

lib/screens/scan_tab.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import '../main.dart';
 import '../scanner_screen.dart';
 import '../services/collection_service.dart';
@@ -12,6 +13,8 @@ class ScanTab extends StatefulWidget {
 }
 
 class ScanTabState extends State<ScanTab> {
+  static const _activeCollectionPrefKey = 'active_collection_id';
+
   bool _isBusy = false;
   List<Collection> _collections = [];
   Collection? _selectedCollection;
@@ -28,12 +31,29 @@ class ScanTabState extends State<ScanTab> {
   Future<void> _loadCollections() async {
     try {
       final list = await CollectionService.getMyCollections();
+      final prefs = await SharedPreferences.getInstance();
+      final persistedId = prefs.getString(_activeCollectionPrefKey);
+
+      Collection? selected;
+      if (persistedId != null) {
+        final matching = list.where((c) => c.id == persistedId);
+        if (matching.isNotEmpty) {
+          selected = matching.first;
+        }
+      }
+
+      selected ??= list.isNotEmpty ? list.first : null;
+
       if (!mounted) return;
       setState(() {
         _collections = list;
-        _selectedCollection = list.isNotEmpty ? list.first : null;
+        _selectedCollection = selected;
         _loadingCollections = false;
       });
+
+      if (selected != null) {
+        await prefs.setString(_activeCollectionPrefKey, selected.id);
+      }
     } catch (e) {
       if (!mounted) return;
       setState(() => _loadingCollections = false);
@@ -133,11 +153,7 @@ class ScanTabState extends State<ScanTab> {
                               ))
                           .toList(),
                       onChanged: (id) {
-                        setState(() {
+                        _setActiveCollection(id);
-                          final matching = _collections.where((c) => c.id == id);
-                          _selectedCollection =
-                              matching.isNotEmpty ? matching.first : null;
-                        });
                       },
                     ),
                   ),
@@ -208,6 +224,17 @@ class ScanTabState extends State<ScanTab> {
     );
   }
 
+  Future<void> _setActiveCollection(String? id) async {
+    if (id == null) return;
+    final matching = _collections.where((c) => c.id == id);
+    if (matching.isEmpty) return;
+
+    setState(() => _selectedCollection = matching.first);
+
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_activeCollectionPrefKey, id);
+  }
+
   Future<void> _openScanner() async {
     await navigatorKey.currentState!.push<void>(
       MaterialPageRoute(

lib/services/collection_service.dart

@@ -259,6 +259,21 @@ class CollectionService {
     required String collectionId,
     required String email,
   }) async {
+    final currentUserId = supabase.auth.currentUser!.id;
+
+    final collection = await supabase
+        .from('collections')
+        .select('owner_id')
+        .eq('id', collectionId)
+        .maybeSingle();
+
+    if (collection == null) {
+      throw Exception('Collection not found.');
+    }
+    if (collection['owner_id'] != currentUserId) {
+      throw Exception('Only the collection owner can invite members.');
+    }
+
     // Call an RPC to look up the user ID by email.
     final result = await supabase.rpc('get_user_id_by_email', params: {
       'lookup_email': email.trim().toLowerCase(),
@@ -271,6 +286,10 @@ class CollectionService {
 
     final userId = result is List ? result.first['id'] as String : result as String;
 
+    if (userId == currentUserId) {
+      throw Exception('You are already in this collection.');
+    }
+
     // Check if already a member.
     final existing = await supabase
         .from('collection_members')
@@ -295,6 +314,35 @@ class CollectionService {
     required String collectionId,
     required String membershipId,
   }) async {
+    final currentUserId = supabase.auth.currentUser!.id;
+
+    final collection = await supabase
+        .from('collections')
+        .select('owner_id')
+        .eq('id', collectionId)
+        .maybeSingle();
+
+    if (collection == null) {
+      throw Exception('Collection not found.');
+    }
+    if (collection['owner_id'] != currentUserId) {
+      throw Exception('Only the collection owner can remove members.');
+    }
+
+    final target = await supabase
+        .from('collection_members')
+        .select('role, user_id')
+        .eq('id', membershipId)
+        .eq('collection_id', collectionId)
+        .maybeSingle();
+
+    if (target == null) {
+      throw Exception('Member not found.');
+    }
+    if (target['role'] == 'owner') {
+      throw Exception('Collection owner cannot be removed.');
+    }
+
     await supabase
         .from('collection_members')
         .delete()
@@ -304,6 +352,21 @@ class CollectionService {
   /// Leave a collection (for non-owners).
   static Future<void> leave(String collectionId) async {
     final userId = supabase.auth.currentUser!.id;
+
+    final membership = await supabase
+        .from('collection_members')
+        .select('role')
+        .eq('collection_id', collectionId)
+        .eq('user_id', userId)
+        .maybeSingle();
+
+    if (membership == null) {
+      throw Exception('You are not a member of this collection.');
+    }
+    if (membership['role'] == 'owner') {
+      throw Exception('Owner cannot leave. Delete the collection instead.');
+    }
+
     await supabase
         .from('collection_members')
         .delete()

pubspec.lock

@@ -841,7 +841,7 @@ packages:
     source: hosted
     version: "0.28.0"
   shared_preferences:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: shared_preferences
       sha256: "2939ae520c9024cb197fc20dee269cd8cdbf564c8b5746374ec6cacdc5169e64"

pubspec.yaml

@@ -45,6 +45,7 @@ dependencies:
   cached_network_image: ^3.4.1
   image: ^4.2.0
   package_info_plus: ^8.1.3
+  shared_preferences: ^2.5.3
 
 dev_dependencies:
   flutter_test: