feat: add Flutter desktop client for Rtty

desktop/test/widget_test.dart

@@ -0,0 +1,15 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:rtty_desktop/main.dart';
+import 'package:rtty_desktop/src/terminal_screen.dart';
+
+void main() {
+  testWidgets('Rtty desktop renders terminal screen', (WidgetTester tester) async {
+    await tester.pumpWidget(const RttyApp());
+
+    // 未连接状态应显示连接提示与连接控制条。
+    expect(find.text('AWAITING CONNECTION'), findsOneWidget);
+    expect(find.text('CONNECT'), findsOneWidget);
+    expect(find.byType(TerminalScreen), findsOneWidget);
+  });
+}