feat(collections): add active scan collection indicator and quick switch

- Load and display persisted active collection in Collections list
- Add Active badge for the currently selected scan destination
- Add quick action to set any collection as active for scanner workflow
- Auto-set active collection when opening a collection details view

lib/screens/collections_screen.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import '../main.dart';
 import '../services/collection_service.dart';
 import '../theme/app_colors.dart';
@@ -14,9 +15,12 @@ class CollectionsScreen extends StatefulWidget {
 }
 
 class CollectionsScreenState extends State<CollectionsScreen> {
+  static const _activeCollectionPrefKey = 'active_collection_id';
+
   List<Collection> _collections = [];
   bool _isLoading = true;
   String? _error;
+  String? _activeCollectionId;
 
   @override
   void initState() {
@@ -35,11 +39,24 @@ class CollectionsScreenState extends State<CollectionsScreen> {
 
     try {
       final list = await CollectionService.getMyCollections();
+      final prefs = await SharedPreferences.getInstance();
+      final persisted = prefs.getString(_activeCollectionPrefKey);
+
+      String? activeId = persisted;
+      if (activeId != null && !list.any((c) => c.id == activeId)) {
+        activeId = list.isNotEmpty ? list.first.id : null;
+      }
+
       if (!mounted) return;
       setState(() {
         _collections = list;
+        _activeCollectionId = activeId;
         _isLoading = false;
       });
+
+      if (activeId != null) {
+        await prefs.setString(_activeCollectionPrefKey, activeId);
+      }
     } catch (e) {
       if (!mounted) return;
       setState(() {
@@ -130,6 +147,7 @@ class CollectionsScreenState extends State<CollectionsScreen> {
   }
 
   void _openCollection(Collection c) {
+    _setActiveCollection(c.id);
     navigatorKey.currentState!.push(
       MaterialPageRoute(
         builder: (_) => GarageScreen(
@@ -149,6 +167,14 @@ class CollectionsScreenState extends State<CollectionsScreen> {
     ).then((_) => _load());
   }
 
+  Future<void> _setActiveCollection(String collectionId) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_activeCollectionPrefKey, collectionId);
+    if (!mounted) return;
+    setState(() => _activeCollectionId = collectionId);
+    showGlobalSnackBar('Active scan collection set.');
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -267,6 +293,8 @@ class CollectionsScreenState extends State<CollectionsScreen> {
                     final c = _collections[index];
                     return _CollectionCard(
                       collection: c,
+                      isActive: _activeCollectionId == c.id,
+                      onSetActive: () => _setActiveCollection(c.id),
                       onTap: () => _openCollection(c),
                       onManage: () => _manageCollection(c),
                     );
@@ -291,11 +319,15 @@ class CollectionsScreenState extends State<CollectionsScreen> {
 
 class _CollectionCard extends StatelessWidget {
   final Collection collection;
+  final bool isActive;
+  final VoidCallback onSetActive;
   final VoidCallback onTap;
   final VoidCallback onManage;
 
   const _CollectionCard({
     required this.collection,
+    required this.isActive,
+    required this.onSetActive,
     required this.onTap,
     required this.onManage,
   });
@@ -377,17 +409,46 @@ class _CollectionCard extends StatelessWidget {
                             ),
                           ),
                         ),
+                        const SizedBox(width: 8),
+                        if (isActive)
+                          Container(
+                            padding: const EdgeInsets.symmetric(
+                                horizontal: 8, vertical: 2),
+                            decoration: BoxDecoration(
+                              color: AppColors.success.withValues(alpha: 0.14),
+                              borderRadius: BorderRadius.circular(6),
+                            ),
+                            child: const Text(
+                              'Active',
+                              style: TextStyle(
+                                fontSize: 11,
+                                fontWeight: FontWeight.w600,
+                                color: AppColors.success,
+                              ),
+                            ),
+                          ),
                       ],
                     ),
                   ],
                 ),
               ),
 
-              // Manage button
-              IconButton(
-                icon: const Icon(Icons.settings_outlined,
-                    color: AppColors.textHint),
-                onPressed: onManage,
+              Column(
+                children: [
+                  IconButton(
+                    tooltip: isActive ? 'Already active for scan' : 'Set active for scan',
+                    icon: Icon(
+                      isActive ? Icons.my_location : Icons.location_searching,
+                      color: isActive ? AppColors.success : AppColors.textHint,
+                    ),
+                    onPressed: isActive ? null : onSetActive,
+                  ),
+                  IconButton(
+                    icon: const Icon(Icons.settings_outlined,
+                        color: AppColors.textHint),
+                    onPressed: onManage,
+                  ),
+                ],
               ),
             ],
           ),