handle Cmd+Q on macOS (#1834)

* feat(video): add Pref.keyboardControl check &  support in video page

- 在 player_focus 处理按键事件前增加 Pref.keyboardControl 检查，允许禁用键盘快捷键
- 修复 Cmd+Q 事件被 Q 键逻辑拦截导致无法退出的问题

* Update player_focus.dart

lib/pages/video/widgets/player_focus.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:io' show exit, Platform;
 import 'dart:math' as math;
 
 import 'package:PiliPlus/pages/common/common_intro_controller.dart';
@@ -93,6 +94,9 @@ class PlayerFocus extends StatelessWidget {
     final isKeyQ = key == LogicalKeyboardKey.keyQ;
     if (isKeyQ || key == LogicalKeyboardKey.keyR) {
       if (HardwareKeyboard.instance.isMetaPressed) {
+        if (isKeyQ && Platform.isMacOS) {
+          exit(0);
+        }
         return true;
       }
       if (event is KeyDownEvent) {