fix: wire backend<->engine data flow so content renders and input works

desktop/lib/src/terminal_screen.dart

@@ -1,3 +1,6 @@
+import 'dart:async';
+import 'dart:typed_data';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_alacritty/flutter_alacritty.dart';
 
@@ -25,6 +28,8 @@ class _TerminalScreenState extends State<TerminalScreen> {
   final FocusNode _focus = FocusNode();
 
   RttyPtyBackend? _backend;
+  StreamSubscription<Uint8List>? _engineOutputSub;
+  StreamSubscription<Uint8List>? _backendOutputSub;
 
   final _host = TextEditingController(text: '127.0.0.1');
   final _port = TextEditingController(text: '8080');
@@ -66,14 +71,22 @@ class _TerminalScreenState extends State<TerminalScreen> {
       _backend = RttyPtyBackend();
     });
 
-    _backend!.onStateChanged = (s) {
+    final engine = _engine!;
+    final backend = _backend!;
+
+    // 双向接线：backend 输出（服务端 ANSI）→ 引擎渲染；
+    // 引擎输出（用户输入）→ backend 发回服务端写入远端 PTY。
+    _engineOutputSub = engine.output.listen(backend.write);
+    _backendOutputSub = backend.output.listen(engine.feed);
+
+    backend.onStateChanged = (s) {
       if (!mounted) return;
       setState(() => _state = s);
       if (s == ConnState.disconnected || s == ConnState.error) {
         _sessionId = null;
       }
     };
-    _backend!.onReady = (id, cols, rows) {
+    backend.onReady = (id, cols, rows) {
       if (!mounted) return;
       setState(() {
         _sessionId = id;
@@ -82,12 +95,17 @@ class _TerminalScreenState extends State<TerminalScreen> {
       });
     };
 
-    await _backend!.connect(host: host, port: port, session: session);
+    await backend.connect(host: host, port: port, session: session);
   }
 
   void _disconnect() => _tearDownSession();
 
   void _tearDownSession() {
+    _engineOutputSub?.cancel();
+    _engineOutputSub = null;
+    _backendOutputSub?.cancel();
+    _backendOutputSub = null;
+
     final b = _backend;
     if (b != null) {
       b.onStateChanged = null;