Initial commit: RustDesk 1.4.7 macOS desktop port

- Filled empty libs/hbb_common/ submodule (cloned from rustdesk/hbb_common)
- Patched Flutter 3.44 / Dart 3.12 compatibility:
  * flutter/lib/generated_bridge.dart: asTypedList with cast<>, DartPort=Int64
  * flutter/lib/common.dart: DialogTheme->DialogThemeData, TabBarTheme->TabBarThemeData
  * flutter/pubspec.yaml: extended_text 14.0.0->15.0.2, google_fonts override 5.0.0
  * flutter/macos/Runner/Configs/Release.xcconfig: EXCLUDED_ARCHS=x86_64
- Build verified: cargo check + cargo build --features flutter + cargo build --release --features flutter
- Verified flutter build macos --debug and --release both produce working .app
- Verified .dmg installer (27MB arm64) created via hdiutil
- Build deps: Xcode 26.5, CocoaPods 1.16.2, Flutter 3.44, VCPKG arm64-osx

flutter/lib/desktop/widgets/titlebar_widget.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+
+const sidebarColor = Color(0xFF0C6AF6);
+const backgroundStartColor = Color(0xFF0583EA);
+const backgroundEndColor = Color(0xFF0697EA);
+
+class DesktopTitleBar extends StatelessWidget {
+  final Widget? child;
+
+  const DesktopTitleBar({Key? key, this.child}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      decoration: const BoxDecoration(
+        gradient: LinearGradient(
+            begin: Alignment.topCenter,
+            end: Alignment.bottomCenter,
+            colors: [backgroundStartColor, backgroundEndColor],
+            stops: [0.0, 1.0]),
+      ),
+      child: Row(
+        children: [
+          Expanded(
+            child: child ?? Offstage(),
+          )
+        ],
+      ),
+    );
+  }
+}