fix: icon permissions and GTK single instance

- Use GTK default flags for single instance
- Fix icon path to absolute /usr/share/icons
- Add postinstall chmod 644 for icon
- Remove Dart-level single instance code

lib/features/window/notifier/window_notifier.dart

@@ -19,11 +19,6 @@ class WindowNotifier extends _$WindowNotifier with AppLogger {
   Future<void> build() async {
     if (!PlatformUtils.isDesktop) return;
 
-    // if (Platform.isWindows) {
-    //   loggy.debug("ensuring single instance");
-    //   await WindowsSingleInstance.ensureSingleInstance([], "Hiddify");
-    // }
-
     await windowManager.ensureInitialized();
     await windowManager.setMinimumSize(minimumWindowSize);
     await windowManager.setSize(defaultWindowSize);