feat: move theme toggle to settings dialog

Move the theme toggle from the main header to the settings dialog for a
cleaner UI and better organization. The new theme selector uses a
button group design consistent with the language settings.

Changes:
- Created ThemeSettings component with three options: Light, Dark, System
- Added ThemeSettings to the General tab in settings dialog
- Removed ModeToggle component from main header
- Added theme-related i18n keys for all options
- Theme selection takes effect immediately without requiring save

Design:
- Uses button group style matching LanguageSettings
- Icons for each theme option (Sun, Moon, Monitor)
- Consistent with app's blue theme for active state
- Smooth transitions and hover effects

This change simplifies the main header and consolidates all appearance
settings in one place, improving the overall user experience.

src/i18n/locales/zh.json

@@ -87,6 +87,12 @@
     "title": "设置",
     "general": "通用",
     "language": "界面语言",
+    "languageHint": "切换后立即预览界面语言，保存后永久生效。",
+    "theme": "外观主题",
+    "themeHint": "选择应用的外观主题，立即生效。",
+    "themeLight": "浅色",
+    "themeDark": "深色",
+    "themeSystem": "跟随系统",
     "importExport": "导入导出配置",
     "exportConfig": "导出配置到文件",
     "selectConfigFile": "选择配置文件",