feat(scanner): add in-scanner active collection switcher

- Add collection dropdown at top of scanner screen
- Wire scanner selection changes back to scan tab active collection state
- Reuse persisted active collection behavior while scanning
- Keep continuous scan loop intact with collection changes applied immediately

lib/scanner_screen.dart

@@ -3,6 +3,7 @@ import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:camera/camera.dart';
 import 'package:google_mlkit_text_recognition/google_mlkit_text_recognition.dart';
+import 'services/collection_service.dart';
 import 'theme/app_colors.dart';
 
 /// Screen that uses the camera to scan text (OCR) from a Hot Wheels package
@@ -11,8 +12,17 @@ import 'theme/app_colors.dart';
 /// The detected ID is returned via Navigator.pop(context, hwId).
 class ScannerScreen extends StatefulWidget {
   final Future<bool> Function(String hwId)? onDetected;
+  final List<Collection> collections;
+  final String? activeCollectionId;
+  final ValueChanged<String>? onCollectionChanged;
 
-  const ScannerScreen({super.key, this.onDetected});
+  const ScannerScreen({
+    super.key,
+    this.onDetected,
+    this.collections = const [],
+    this.activeCollectionId,
+    this.onCollectionChanged,
+  });
 
   @override
   State<ScannerScreen> createState() => _ScannerScreenState();
@@ -26,6 +36,7 @@ class _ScannerScreenState extends State<ScannerScreen> {
   String? _lastDetected;
   bool _scanAccepted = false;
   String _statusText = 'Ready to scan';
+  String? _activeCollectionId;
 
   // Matches typical Hot Wheels model IDs: 2–5 uppercase letters followed by
   // 2–4 digits, e.g. JKF21, HCV73, GRX33, FYD83.
@@ -34,10 +45,17 @@ class _ScannerScreenState extends State<ScannerScreen> {
   @override
   void initState() {
     super.initState();
+    _activeCollectionId = widget.activeCollectionId;
     _textRecognizer = TextRecognizer();
     _initCamera();
   }
 
+  void _handleCollectionChanged(String? value) {
+    if (value == null) return;
+    setState(() => _activeCollectionId = value);
+    widget.onCollectionChanged?.call(value);
+  }
+
   Future<void> _initCamera() async {
     final cameras = await availableCameras();
     if (cameras.isEmpty) {
@@ -188,6 +206,37 @@ class _ScannerScreenState extends State<ScannerScreen> {
       ),
       body: Column(
         children: [
+          if (widget.collections.isNotEmpty)
+            Container(
+              width: double.infinity,
+              padding: const EdgeInsets.fromLTRB(12, 10, 12, 8),
+              color: Colors.black.withValues(alpha: 0.55),
+              child: DropdownButtonHideUnderline(
+                child: DropdownButton<String>(
+                  isExpanded: true,
+                  value: _activeCollectionId,
+                  dropdownColor: AppColors.navy,
+                  iconEnabledColor: Colors.white,
+                  style: const TextStyle(color: Colors.white),
+                  hint: const Text(
+                    'Select collection',
+                    style: TextStyle(color: Colors.white70),
+                  ),
+                  items: widget.collections
+                      .map(
+                        (c) => DropdownMenuItem<String>(
+                          value: c.id,
+                          child: Text(
+                            c.name,
+                            overflow: TextOverflow.ellipsis,
+                          ),
+                        ),
+                      )
+                      .toList(),
+                  onChanged: _isBusy ? null : _handleCollectionChanged,
+                ),
+              ),
+            ),
           Container(
             width: double.infinity,
             padding: const EdgeInsets.symmetric(horizontal: 14, vertical: 8),

lib/screens/scan_tab.dart

@@ -239,6 +239,9 @@ class ScanTabState extends State<ScanTab> {
     await navigatorKey.currentState!.push<void>(
       MaterialPageRoute(
         builder: (_) => ScannerScreen(
+          collections: _collections,
+          activeCollectionId: _selectedCollection?.id,
+          onCollectionChanged: _setActiveCollection,
           onDetected: (hwId) => _processHwId(hwId),
         ),
       ),