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/i18n/locales/en/setting.json

@@ -9,7 +9,24 @@
     "dark": "Dark Theme",
     "light": "Light Theme",
     "system": "System Theme",
-    "primaryColor": "Primary Color"
+    "primaryColor": "Primary Color",
+    "description": "Customize the look and feel of the application",
+    "themeMode": "Theme Mode",
+    "themeModeDescription": "Choose your preferred theme mode, or follow system settings to switch automatically.",
+    "lightLabel": "Light",
+    "lightDescription": "Bright and clear",
+    "darkLabel": "Dark",
+    "darkDescription": "Eye comfort",
+    "systemLabel": "Follow system",
+    "systemDescription": "Auto switch",
+    "currentTheme": "Current theme",
+    "followingSystem": "Following system theme ({{theme}})",
+    "setToLight": "Set to light theme",
+    "setToDark": "Set to dark theme",
+    "preview": "Preview",
+    "previewDescription": "View the current theme effect",
+    "lightPreview": "Light preview",
+    "darkPreview": "Dark preview"
   },
   "appearance": {
     "fontSize": "Font Size",

src/i18n/locales/zh/setting.json

@@ -9,7 +9,24 @@
     "dark": "深色主题",
     "light": "浅色主题",
     "system": "跟随系统",
-    "primaryColor": "主题颜色"
+    "primaryColor": "主题颜色",
+    "description": "自定义应用程序的外观和风格",
+    "themeMode": "主题模式",
+    "themeModeDescription": "选择您偏好的主题模式，或跟随系统设置自动切换。",
+    "lightLabel": "浅色",
+    "lightDescription": "明亮清晰",
+    "darkLabel": "深色",
+    "darkDescription": "护眼舒适",
+    "systemLabel": "跟随系统",
+    "systemDescription": "自动切换",
+    "currentTheme": "当前主题",
+    "followingSystem": "跟随系统主题 ({{theme}})",
+    "setToLight": "已设置为浅色主题",
+    "setToDark": "已设置为深色主题",
+    "preview": "预览",
+    "previewDescription": "查看当前主题效果",
+    "lightPreview": "浅色预览",
+    "darkPreview": "深色预览"
   },
   "appearance": {
     "fontSize": "字体大小",