feat: add '复制标签' option to terminal tab right-click menu

User asked: '标签使用鼠标右键应该有复制标签的功能'

In ui/terminal_tab_widget.py _on_tab_context_menu:
- Added '📋 复制标签' action between '重命名' and '关闭标签'
- New method _duplicate_tab(idx): creates a new terminal tab for
  the same host (reuses SSH connection, opens independent shell
  channel via paramiko invoke_shell)
- New tab title = '<src_title> (副本)' with ⚪ state dot
- If host not connected: duplicate creates a placeholder tab
- If host connected: duplicate immediately opens a new shell

Verified: 3 tabs after duplicating 'test-server' (欢迎 + ⚪ test-server
+ ⚪ test-server (副本))

All tests still pass.

ui/terminal_tab_widget.py

@@ -231,6 +231,7 @@ class TerminalTabWidget(QWidget):
         menu = QMenu(self)
         panel = self._panel_at(idx)
         a_rename = menu.addAction("重命名")
+        a_duplicate = menu.addAction("📋 复制标签")
         a_close = menu.addAction("关闭标签")
         a_close_others = menu.addAction("关闭其他")
         if panel and panel._connected:
@@ -242,6 +243,8 @@ class TerminalTabWidget(QWidget):
         chosen = menu.exec_(self.tabs.tabBar().mapToGlobal(pos))
         if chosen == a_rename:
             self._on_tab_double_clicked(idx)
+        elif chosen == a_duplicate:
+            self._duplicate_tab(idx)
         elif chosen == a_close:
             self._close_tab(idx)
         elif chosen == a_close_others:
@@ -250,6 +253,29 @@ class TerminalTabWidget(QWidget):
             if panel and panel.conn:
                 panel._reopen_shell()
 
+    def _duplicate_tab(self, idx: int):
+        """复制标签：新建一个同主机的终端标签（独立 shell 通道）"""
+        src_panel = self._panel_at(idx)
+        if src_panel is None:
+            return
+        host_id = self._host_id_for_panel(src_panel)
+        # 决定新标签的初始连接
+        conn = self.manager.get_connection(host_id) if host_id else None
+        # 新建标签（open_terminal 会 attach conn）
+        new_idx = self.open_terminal(host_id or "", conn=conn)
+        # 用源标签的当前标题做新标签名（去掉状态点前缀）
+        src_title = self.tabs.tabText(idx)
+        for prefix in ("⚪ ", "🟢 ", "🔴 ", "🟡 "):
+            if src_title.startswith(prefix):
+                src_title = src_title[len(prefix):]
+                break
+        # 加 " (副本)" 后缀
+        self.tabs.setTabText(new_idx, f"⚪ {src_title} (副本)")
+        self.tabs.setCurrentIndex(new_idx)
+        # 如果已连接，给用户一个小提示：每个副本是独立 shell
+        if conn:
+            self.statusBar_msg = f"已创建副本（独立 shell）" if hasattr(self, "statusBar_msg") else None
+
     def _close_tab(self, idx: int):
         panel = self._panel_at(idx)
         if panel and panel._connected: