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.

src/styles/index.css

@@ -1,4 +1,8 @@
 @import "tailwindcss";
+
+/* 启用 dark 变体，基于 .dark 类 */
+@custom-variant dark (&:where(.dark, .dark *));
+
 @import "./theme/index.css";
 @import "./tailwind.css";
 @plugin "@tailwindcss/typography";