feat: add Flutter desktop client for Rtty

desktop/lib/main.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+import 'src/terminal_screen.dart';
+import 'src/theme.dart';
+
+void main() {
+  WidgetsFlutterBinding.ensureInitialized();
+
+  // 锁定竖排字符方向（终端场景无需旋转）。
+  SystemChrome.setPreferredOrientations(DeviceOrientation.values);
+
+  runApp(const RttyApp());
+}
+
+class RttyApp extends StatelessWidget {
+  const RttyApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Rtty Desktop',
+      debugShowCheckedModeBanner: false,
+      theme: RttyTheme.app(),
+      home: const TerminalScreen(),
+    );
+  }
+}