feat(theme): implement comprehensive theme management system

Add full theme support with light, dark, and system modes, including:
- Theme store using Pinia for state management
- useTheme composable for reactive theme handling
- Theme setting UI in settings page
- Enhanced CSS variable system with Tailwind integration
- IPC communication for theme persistence
- Internationalization support for theme texts
- System theme detection and auto-switching

The implementation follows ClawX's architecture while adapting to Vue 3 and zn-ai's existing infrastructure.

global.d.ts

@@ -40,6 +40,19 @@ declare global {
       params: [message: string]
       return: void
     }
+    // 主题事件
+    [IPC_EVENTS.THEME_MODE_UPDATED]: {
+      params: [isDark: boolean]
+      return: void
+    }
+    [IPC_EVENTS.SET_THEME_MODE]: {
+      params: [theme: ThemeMode]
+      return: Promise<boolean>  // 返回 isDark
+    }
+    [IPC_EVENTS.GET_THEME_MODE]: {
+      params: []
+      return: Promise<ThemeMode>
+    }
   }
 
   type TabId = string