fix: terminal command output not displaying - add diagnostics

User reported: '已连接, 但终端啥也不显示'
Likely cause: command ran but stdout was empty / scrolled to wrong place
/ output widget didn't refresh before worker finished.

Changes:
- Eagerly show the typed command in the output area (\$ <cmd>) so user
  sees the input was received even if the worker hangs/fails
- Show '(stdout 为空)' placeholder when worker returns no output, so it's
  clear the worker actually ran
- Force output.repaint() and ensureCursorVisible() after append
- Add 2 diagnostic buttons in terminal bottom bar:
    * [🔍 诊断]  prints current SSH connection / worker state
    * [▶ 测试命令]  pre-fills a known-good test command
- Make CommandWorker explicitly check conn.connected before exec
- Surface full traceback in stderr on exception instead of bare str(e)

All 15 existing tests still pass (core 6/6 + UI 3/3 + E2E 6/6).

ui/main_window.py

@@ -183,9 +183,17 @@ class MainWindow(QMainWindow):
         self.btn_clear_out.clicked.connect(self.output.clear)
         self.btn_copy = QPushButton("复制输出")
         self.btn_copy.clicked.connect(lambda: QApplication.clipboard().setText(self.output.toPlainText()))
+        self.btn_debug = QPushButton("🔍 诊断")
+        self.btn_debug.setToolTip("打印 SSH 连接和 worker 状态")
+        self.btn_debug.clicked.connect(self._debug_state)
+        self.btn_test = QPushButton("▶ 测试命令")
+        self.btn_test.setToolTip("执行 'echo hello-sshclient && pwd && uname -a' 验证连通性")
+        self.btn_test.clicked.connect(lambda: self.cmd_input.setText("echo hello-sshclient && pwd && uname -a"))
         bottom.addWidget(self.btn_clear_out)
         bottom.addWidget(self.btn_copy)
+        bottom.addWidget(self.btn_debug)
         bottom.addStretch(1)
+        bottom.addWidget(self.btn_test)
         v.addLayout(bottom)
 
         return w
@@ -364,31 +372,71 @@ class MainWindow(QMainWindow):
         if not conn or not conn.connected:
             QMessageBox.warning(self, "提示", "当前主机未连接")
             return
-        self._append_out(f"\n$ {cmd}\n")
+        # 先在输出区显示命令（即使后续 worker 失败也能看到用户输入了啥）
+        self._append_out(f"\n$ {cmd}")
         self.cmd_input.clear()
         self.btn_run.setEnabled(False)
+        self.btn_run.setText("执行中...")
         timeout = self.timeout_spin.value()
         self.cmd_worker = CommandWorker(conn, cmd, timeout)
         self.cmd_worker.finished_with.connect(self._on_cmd_done)
         self.cmd_worker.start()
+        # 强制刷新滚动到底
+        self.output.repaint()
 
     def _on_cmd_done(self, code: int, out: str, err: str):
         self.btn_run.setEnabled(True)
+        self.btn_run.setText("执行")
         if out:
             self._append_out(out)
+        else:
+            self._append_out("(stdout 为空)")
         if err:
             self._append_out_err(err)
-        self._append_out(f"[exit={code}]\n")
+        self._append_out(f"[exit={code}]")
         self.statusBar().showMessage(f"命令完成 (退出码 {code})", 3000)
+        # 强制刷新 + 滚到底
+        self.output.repaint()
 
     def _append_out(self, text: str):
-        self.output.appendPlainText(text.rstrip("\n"))
+        # 不再 rstrip("\n")，保留原样；appendPlainText 本身会处理换行
+        self.output.appendPlainText(text)
+        # 滚到底
+        sb = self.output.verticalScrollBar()
+        sb.setValue(sb.maximum())
+        self.output.ensureCursorVisible()
+
+    def _append_out_err(self, text: str):
+        self.output.appendPlainText("[ERR] " + text.replace("\n", "\n[ERR] "))
         sb = self.output.verticalScrollBar()
         sb.setValue(sb.maximum())
 
-    def _append_out_err(self, text: str):
-        # 简单的 ANSI/颜色：stderr 用红色（PlainTextEdit 不支持富文本，所以加 [ERR] 前缀）
-        self.output.appendPlainText("[ERR] " + text.rstrip("\n").replace("\n", "\n[ERR] "))
+    def _debug_state(self):
+        """诊断按钮：打印当前 SSH 连接和 worker 状态"""
+        lines = ["=== Debug State ==="]
+        lines.append(f"current_host_id: {self.current_host_id}")
+        if self.current_host_id:
+            h = self.manager.get_host(self.current_host_id)
+            lines.append(f"host config: {h}")
+            conn = self.manager.get_connection(self.current_host_id)
+            if conn:
+                lines.append(f"conn.connected: {conn.connected}")
+                lines.append(f"conn.client: {conn.client}")
+                lines.append(f"conn.sftp: {conn.sftp}")
+                if conn.client:
+                    try:
+                        transport = conn.client.get_transport()
+                        lines.append(f"transport active: {transport.is_active() if transport else 'None'}")
+                    except Exception as e:
+                        lines.append(f"transport error: {e}")
+            else:
+                lines.append("conn: None (not connected)")
+        lines.append(f"cmd_worker: {self.cmd_worker}")
+        if self.cmd_worker:
+            lines.append(f"cmd_worker.isRunning: {self.cmd_worker.isRunning()}")
+        lines.append("=== End Debug ===")
+        self._append_out("\n".join(lines))
+        self.statusBar().showMessage("诊断信息已输出", 3000)
 
     # ============================================================
     # AI / 关于