feat: command snippets + UI test coverage for new features

- core/snippets.py: SnippetManager with 20 prebuilt sysadmin commands,
  CRUD + reorder, persisted to ~/.sshclient/snippets.json
- ui/snippet_dialog.py: SnippetDialog (add/edit) + SnippetManagerDialog
  (table view with add/edit/delete/move up/down)
- ui/terminal_panel.py: snippet combo box in toolbar, send_command()
  method to inject commands into interactive shell, snippet manager button
- README.md: updated feature table, usage section, project structure
- test_ui.py: added [2/4] checks for sparkline/snippets/theme,
  added [4/4] theme toggle verification

test_ui.py

@@ -33,13 +33,21 @@ def main():
     assert w.tabs.count() == 4, f"应有 4 个 Tab，实际 {w.tabs.count()}"
     print(f"  ✓ 主窗口创建，Tab 数量 = {w.tabs.count()}")
 
-    print("[2/3] 验证子组件")
+    print("[2/4] 验证子组件")
     assert isinstance(w.file_browser, FileBrowser)
     assert isinstance(w.monitor, MonitorPanel)
     assert isinstance(w.ai_panel, AIChatPanel)
+    # 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"
+    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("[3/3] 验证对话框")
+    print("[3/4] 验证对话框")
     # 主机对话框
     hd = HostDialog(current={"name": "test", "host": "1.2.3.4", "port": 22,
                               "username": "u", "password": "p", "key_path": ""})
@@ -56,10 +64,23 @@ def main():
     cd.close()
     print(f"  ✓ AIConfigDialog + {len(PRESETS)} 个预设")
 
-    # 不真正 show，只确保不崩
+    print("[4/4] 验证主题切换")
+    from ui.theme import get_theme, set_theme, get_qss
+    # 切换到暗色
+    w.act_dark.setChecked(True)
+    w._toggle_theme()
+    assert get_theme() == "dark"
+    qss = app.styleSheet()
+    assert len(qss) > 100, "QSS 应该有内容"
+    # 切回亮色
+    w.act_dark.setChecked(False)
+    w._toggle_theme()
+    assert get_theme() == "light"
+    print(f"  ✓ 暗/亮主题切换正常 (dark QSS={len(get_qss('dark'))} chars)")
+
+    # 清理
     w.close()
     print("\n所有 UI 组件加载正常 ✓")
-    # 不进入事件循环，强制退出
     QTimer.singleShot(0, app.quit)
     app.exec_()