refactor(ui): share android keyboard-mode system-back guard

lib/focus/input_mode_tracker.dart

@@ -1,3 +1,5 @@
+import 'dart:io';
+
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
@@ -40,6 +42,13 @@ class InputModeTracker extends StatefulWidget {
     return of(context) == InputMode.keyboard;
   }
 
+  /// Whether system back must be blocked because the dpad key handler owns
+  /// back navigation: on Android in keyboard mode (TV/gamepad), letting the
+  /// system back through as well double-pops the route.
+  static bool shouldBlockSystemBack(BuildContext context) {
+    return Platform.isAndroid && isKeyboardMode(context);
+  }
+
   @override
   State<InputModeTracker> createState() => _InputModeTrackerState();
 }