feat: dark theme toggle + CPU/memory sparkline trend charts

- ui/theme.py: light/dark QSS stylesheets, persisted to ~/.sshclient/settings.json
- main_window.py: View menu with dark theme checkbox, applies on startup
- ui/widgets.py: SparklineChart widget for mini trend visualization
- MonitorPanel: CPU and memory sparkline charts showing last 60 samples
- Charts cleared on stop/switch host

ui/main_window.py

@@ -22,6 +22,7 @@ from .workers import ConnectWorker
 from .widgets import FileBrowser, MonitorPanel, AIChatPanel
 from .config_dialog import AIConfigDialog
 from .terminal_panel import TerminalPanel
+from .theme import get_qss, get_theme, set_theme
 
 
 APP_NAME = "SSHClient"
@@ -43,6 +44,7 @@ class MainWindow(QMainWindow):
         self._build_ui()
         self._build_menu()
         self._build_statusbar()
+        self._apply_theme()
         self._load_hosts_to_list()
 
     # ============================================================
@@ -144,6 +146,12 @@ class MainWindow(QMainWindow):
         act_clear = QAction("清空 AI 对话", self)
         act_clear.triggered.connect(lambda: self.ai_panel._clear())
         m_ai.addAction(act_clear)
+        # 视图
+        m_view = menubar.addMenu("视图(&V)")
+        self.act_dark = QAction("🌙 暗色主题", self, checkable=True)
+        self.act_dark.setChecked(get_theme() == "dark")
+        self.act_dark.triggered.connect(self._toggle_theme)
+        m_view.addAction(self.act_dark)
         # 帮助
         m_help = menubar.addMenu("帮助(&H)")
         act_about = QAction("关于", self)
@@ -301,6 +309,18 @@ class MainWindow(QMainWindow):
     # ============================================================
     # AI / 关于
     # ============================================================
+    def _apply_theme(self):
+        theme = get_theme()
+        qss = get_qss(theme)
+        QApplication.instance().setStyleSheet(qss)
+
+    def _toggle_theme(self):
+        new_theme = "dark" if self.act_dark.isChecked() else "light"
+        set_theme(new_theme)
+        self._apply_theme()
+        label = "暗色" if new_theme == "dark" else "亮色"
+        self.statusBar().showMessage(f"已切换到{label}主题", 3000)
+
     def _show_ai_config(self):
         dlg = AIConfigDialog(self.agent, self)
         if dlg.exec_():