fix(nav): preserve HTPC escape behavior

Use Flutter's supported desktop application-exit API so the second root Back press closes Plezy on Windows. Preserve fullscreen when video player navigation is enabled while keeping fullscreen-first Escape behavior for normal desktop use.

close #1582

lib/services/app_exit_service.dart

@@ -1,9 +1,12 @@
 import 'dart:io' show Platform;
+import 'dart:ui' as ui;
 
 import 'package:flutter/services.dart';
 
 import '../utils/platform_detector.dart';
 
+typedef AppExitApplication = Future<ui.AppExitResponse> Function(ui.AppExitType exitType, int exitCode);
+
 class AppExitService {
   static const bool _tvosBuild = bool.fromEnvironment('TVOS_BUILD');
   static const MethodChannel _channel = MethodChannel('com.plezy/app_exit');
@@ -12,7 +15,7 @@ class AppExitService {
   ///
   /// tvOS has no public API for force-quitting or going Home, so callers that
   /// handle a physical back/Menu key should let the event continue instead.
-  static Future<bool> requestExit() async {
+  static Future<bool> requestExit({AppExitApplication? exitApplicationForTesting}) async {
     if (_tvosBuild || PlatformDetector.isAppleTV()) return false;
 
     if (Platform.isAndroid) {
@@ -27,6 +30,14 @@ class AppExitService {
       }
     }
 
+    if (PlatformDetector.isDesktopOS()) {
+      final exitApplication =
+          exitApplicationForTesting ??
+          (exitType, exitCode) => ServicesBinding.instance.exitApplication(exitType, exitCode);
+      final response = await exitApplication(ui.AppExitType.required, 0);
+      return response == ui.AppExitResponse.exit;
+    }
+
     await SystemNavigator.pop();
     return true;
   }