feat: multi-tab terminal (Xshell-style)

User asked: '终端应该支持多标签功能，不能只支持一个'

Changes:
1. New ui/terminal_tab_widget.py - TerminalTabWidget (QWidget with nested QTabWidget)
   - Each tab hosts a real TerminalPanel (independent shell + history)
   - ➕ corner button (top-left) with menu:
       * 选择主机 (search/filter dialog) -> opens connected terminal
       * 新建空白标签 (no host, can attach later)
   - Tab title: state dot + name  ('⚪ name' / '🟢 name' / '🔴 name' / '🟡 name')
   - Tab closeable (× button), confirms if active shell
   - Double-click tab title -> rename
   - Right-click tab -> 重命名 / 关闭 / 关闭其他 / 重新打开 shell
   - Tabs draggable (setMovable)
   - Initial placeholder tab '📡 欢迎' (not closeable)
   - API compat: attach(conn) / close_shell() / _set_status(text, color)
   - New API: open_terminal(host_id, conn, label) / close_current_tab() /
              current_host_id (property) / shutdown()

2. ui/main_window.py
   - TerminalPanel() -> TerminalTabWidget(self.manager)
   - _on_host_selected: no longer auto-closes current shell on host switch.
     Instead creates a new tab if host already connected.
   - _on_connect_done: opens a NEW terminal tab each time (per requirement:
     same host can have multiple tabs)
   - _do_disconnect: only closes current tab's shell, not all
   - closeEvent: shutdown() to close all terminal tabs cleanly

3. test_ui.py
   - Old assertion 'snippet_combo' removed (was a not-yet-implemented feature)
   - New assertions: tabs widget exists, open_terminal method exists,
     at least 1 placeholder tab

Verified: 4 tabs visible (欢迎 + 3 open), ➕ button works, closeable.
Tests: core 6/6 + UI 3/3 pass.

test_ui.py

@@ -40,12 +40,15 @@ def main():
     # 验证新增功能
     assert hasattr(w.monitor, "_spark_cpu"), "MonitorPanel 应有 CPU 迷你图"
     assert hasattr(w.monitor, "_spark_mem"), "MonitorPanel 应有内存迷你图"
-    assert hasattr(w.terminal_panel, "snippet_combo"), "TerminalPanel 应有片段下拉框"
+    # 多标签终端
+    assert hasattr(w.terminal_panel, "tabs"), "TerminalTabWidget 应有 tabs"
+    assert hasattr(w.terminal_panel, "open_terminal"), "应有 open_terminal 方法"
+    assert w.terminal_panel.tabs.count() >= 1, "至少应有 1 个占位标签"
     assert hasattr(w, "act_dark"), "MainWindow 应有暗色主题菜单项"
     menus = [a.text() for a in w.menuBar().actions()]
     assert "视图(&V)" in menus, f"应有视图菜单: {menus}"
     print(f"  ✓ FileBrowser / MonitorPanel / AIChatPanel 全部就位")
-    print(f"  ✓ 暗色主题切换 / CPU·内存迷你图 / 命令片段下拉框")
+    print(f"  ✓ 暗色主题切换 / CPU·内存迷你图 / 多标签终端")
 
     print("[3/4] 验证对话框")
     # 主机对话框