fix: extract manual entry dialog into StatefulWidget

Fixes TextEditingController used after being disposed  the controller
was being manually disposed while the dialog's TextField still held a
reference during the pop animation. Now the dialog is its own
StatefulWidget so dispose() is called automatically by the framework
at the right time.

lib/scanner_screen.dart

@@ -212,42 +212,63 @@ class _ScannerScreenState extends State<ScannerScreen> {
 
   /// Fallback: let the user type the HW ID manually.
   Future<void> _showManualEntry(BuildContext context) async {
-    final controller = TextEditingController();
-
     final result = await showDialog<String>(
       context: context,
-      builder: (ctx) => AlertDialog(
+      builder: (_) => const _ManualEntryDialog(),
+    );
+
+    if (result != null && context.mounted) {
+      Navigator.of(context).pop(result);
+    }
+  }
+}
+
+// ── Manual Entry Dialog ───────────────────────────────────────────────
+class _ManualEntryDialog extends StatefulWidget {
+  const _ManualEntryDialog();
+
+  @override
+  State<_ManualEntryDialog> createState() => _ManualEntryDialogState();
+}
+
+class _ManualEntryDialogState extends State<_ManualEntryDialog> {
+  final _controller = TextEditingController();
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  void _submit() {
+    final value = _controller.text.trim().toUpperCase();
+    if (value.isNotEmpty) Navigator.of(context).pop(value);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return AlertDialog(
       title: const Text('Enter HW ID'),
       content: TextField(
-          controller: controller,
+        controller: _controller,
         autofocus: true,
         textCapitalization: TextCapitalization.characters,
         decoration: const InputDecoration(
           hintText: 'e.g. JKF21',
           border: OutlineInputBorder(),
         ),
+        onSubmitted: (_) => _submit(),
       ),
       actions: [
         TextButton(
-            onPressed: () => Navigator.of(ctx).pop(),
+          onPressed: () => Navigator.of(context).pop(),
           child: const Text('Cancel'),
         ),
         ElevatedButton(
-            onPressed: () {
-              final value = controller.text.trim().toUpperCase();
-              if (value.isNotEmpty) Navigator.of(ctx).pop(value);
-            },
+          onPressed: _submit,
           child: const Text('OK'),
         ),
       ],
-      ),
     );
-
-    if (result != null && context.mounted) {
-      controller.dispose();
-      Navigator.of(context).pop(result);
-    } else {
-      controller.dispose();
-    }
   }
 }