fix: harden scanner camera preview recovery

lib/scanner_screen.dart

@@ -30,7 +30,8 @@ class ScannerScreen extends StatefulWidget {
   State<ScannerScreen> createState() => _ScannerScreenState();
 }
 
-class _ScannerScreenState extends State<ScannerScreen> {
+class _ScannerScreenState extends State<ScannerScreen>
+  with WidgetsBindingObserver {
   static const _autoScanTickInterval = Duration(milliseconds: 700);
   static const _scanCooldownSuccess = Duration(milliseconds: 1500);
   static const _scanCooldownNoMatch = Duration(milliseconds: 2200);
@@ -48,48 +49,103 @@ class _ScannerScreenState extends State<ScannerScreen> {
   bool _autoScanEnabled = false;
   Timer? _autoScanTimer;
   DateTime _nextScanAllowedAt = DateTime.fromMillisecondsSinceEpoch(0);
+  bool _isInitializingCamera = false;
+  String? _cameraError;
 
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addObserver(this);
     _activeCollectionId = widget.activeCollectionId;
     _autoScanEnabled = widget.onDetected != null;
     _textRecognizer = TextRecognizer();
     _initCamera();
   }
 
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.inactive ||
+        state == AppLifecycleState.paused ||
+        state == AppLifecycleState.detached) {
+      _disposeCamera();
+      return;
+    }
+
+    if (state == AppLifecycleState.resumed) {
+      _initCamera(force: true);
+    }
+  }
+
   void _handleCollectionChanged(String? value) {
     if (value == null) return;
     setState(() => _activeCollectionId = value);
     widget.onCollectionChanged?.call(value);
   }
 
-  Future<void> _initCamera() async {
+  Future<void> _initCamera({bool force = false}) async {
-    final cameras = await availableCameras();
+    if (_isInitializingCamera) return;
-    if (cameras.isEmpty) {
+    if (_cameraReady && !force) return;
-      if (!mounted) return;
+
-      ScaffoldMessenger.of(context).showSnackBar(
+    _isInitializingCamera = true;
-        const SnackBar(content: Text('No camera available')),
+    try {
+      await _disposeCamera();
+
+      final cameras = await availableCameras();
+      if (cameras.isEmpty) {
+        throw Exception('No camera available');
+      }
+
+      final backCamera = cameras.firstWhere(
+        (c) => c.lensDirection == CameraLensDirection.back,
+        orElse: () => cameras.first,
       );
-      return;
+
+      final controller = CameraController(
+        backCamera,
+        ResolutionPreset.medium,
+        enableAudio: false,
+      );
+
+      await controller.initialize();
+      if (!mounted) {
+        await controller.dispose();
+        return;
+      }
+
+      _cameraController = controller;
+      setState(() {
+        _cameraReady = true;
+        _cameraError = null;
+        _statusText = 'Ready to scan';
+      });
+      _startAutoScanLoop();
+    } catch (e) {
+      if (!mounted) return;
+      setState(() {
+        _cameraReady = false;
+        _cameraError = 'Camera unavailable. Please retry.';
+        _statusText = 'Camera unavailable';
+      });
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text('Camera init failed: $e'),
+          backgroundColor: Colors.red,
+        ),
+      );
+    } finally {
+      _isInitializingCamera = false;
     }
+  }
 
-    // Use the first back-facing camera.
+  Future<void> _disposeCamera() async {
-    final backCamera = cameras.firstWhere(
+    _autoScanTimer?.cancel();
-      (c) => c.lensDirection == CameraLensDirection.back,
+    _autoScanTimer = null;
-      orElse: () => cameras.first,
+    final controller = _cameraController;
-    );
+    _cameraController = null;
-
+    _cameraReady = false;
-    _cameraController = CameraController(
+    if (controller != null) {
-      backCamera,
+      await controller.dispose();
-      ResolutionPreset.medium,
+    }
-      enableAudio: false,
-    );
-
-    await _cameraController!.initialize();
-    if (!mounted) return;
-    setState(() => _cameraReady = true);
-    _startAutoScanLoop();
   }
 
   void _startAutoScanLoop() {
@@ -210,8 +266,13 @@ class _ScannerScreenState extends State<ScannerScreen> {
 
   @override
   void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
     _autoScanTimer?.cancel();
-    _cameraController?.dispose();
+    final controller = _cameraController;
+    _cameraController = null;
+    if (controller != null) {
+      controller.dispose();
+    }
     _textRecognizer.close();
     super.dispose();
   }
@@ -302,7 +363,32 @@ class _ScannerScreenState extends State<ScannerScreen> {
           ),
           // ── Camera preview ──
           Expanded(
-            child: _cameraReady
+            child: _cameraError != null
+                ? Center(
+                    child: Padding(
+                      padding: const EdgeInsets.symmetric(horizontal: 24),
+                      child: Column(
+                        mainAxisSize: MainAxisSize.min,
+                        children: [
+                          const Icon(Icons.camera_alt_outlined,
+                              size: 46, color: AppColors.error),
+                          const SizedBox(height: 12),
+                          Text(
+                            _cameraError!,
+                            textAlign: TextAlign.center,
+                            style: const TextStyle(color: AppColors.textSecondary),
+                          ),
+                          const SizedBox(height: 12),
+                          OutlinedButton.icon(
+                            onPressed: () => _initCamera(force: true),
+                            icon: const Icon(Icons.refresh),
+                            label: const Text('Retry Camera'),
+                          ),
+                        ],
+                      ),
+                    ),
+                  )
+                : _cameraReady
                 ? Stack(
                     fit: StackFit.expand,
                     children: [
@@ -349,7 +435,7 @@ class _ScannerScreenState extends State<ScannerScreen> {
                       ),
                     ],
                   )
-                : const Center(child: CircularProgressIndicator()),
+                    : const Center(child: CircularProgressIndicator()),
           ),
 
           // ── Detected ID confirmation area ──
@@ -440,7 +526,7 @@ class _ScannerScreenState extends State<ScannerScreen> {
                             _isBusy ? 'Scanning…' : 'Capture & Scan',
                             style: const TextStyle(color: Colors.white, fontWeight: FontWeight.w600),
                           ),
-                          onPressed: _isBusy ? null : _captureAndScan,
+                          onPressed: _isBusy || !_cameraReady ? null : _captureAndScan,
                           style: ElevatedButton.styleFrom(
                             backgroundColor: Colors.transparent,
                             shadowColor: Colors.transparent,