fix: restore hardware key focus after lock/unlock in boxing screen

Add WidgetsBindingObserver lifecycle hook to re-request focus
when app resumes, fixing P2 key not working after screen lock.

lib/pages/boxing_page.dart

@@ -83,7 +83,7 @@ class BoxingPage extends StatefulWidget {
   State<BoxingPage> createState() => _BoxingPageState();
 }
 
-class _BoxingPageState extends State<BoxingPage> {
+class _BoxingPageState extends State<BoxingPage> with WidgetsBindingObserver {
   static const _androidKeyCodeButtonR1 = 103;
 
   final _scannerService = ScannerService();
@@ -147,6 +147,7 @@ class _BoxingPageState extends State<BoxingPage> {
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addObserver(this);
     final arguments = widget.arguments;
     if (arguments != null) {
       _mode = arguments.initialMode;
@@ -163,6 +164,7 @@ class _BoxingPageState extends State<BoxingPage> {
 
   @override
   void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
     _boxNoController.removeListener(_onBoxNoTextChanged);
     _scanSubscription?.cancel();
     _focusNode.dispose();
@@ -180,6 +182,13 @@ class _BoxingPageState extends State<BoxingPage> {
     }
   }
 
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.resumed) {
+      _requestFocus();
+    }
+  }
+
   void _onKeyEvent(KeyEvent event) {
     if (event is! KeyDownEvent || _isAutoProcessing) return;
     if (_isModeSwitchKey(event)) {