feat: integrate language switcher into settings with modern segment control UI

- Move language switcher from header to settings modal for better organization
- Implement modern segment control UI instead of radio buttons for language selection
- Add language preference persistence in localStorage and backend settings
- Support instant language preview with cancel/revert functionality
- Remove standalone LanguageSwitcher component
- Improve initial language detection logic (localStorage -> browser -> default)
- Add proper i18n keys for language settings UI text

src/i18n/locales/zh.json

@@ -62,6 +62,11 @@
   },
   "settings": {
     "title": "设置",
+    "general": "通用",
+    "language": "界面语言",
+    "languageDescription": "选择 CC Switch 的显示语言，下次启动会自动记住你的偏好。",
+    "languageOptionChinese": "中文",
+    "languageOptionEnglish": "English",
     "windowBehavior": "窗口行为",
     "minimizeToTray": "关闭时最小化到托盘",
     "minimizeToTrayDescription": "勾选后点击关闭按钮会隐藏到系统托盘，取消则直接退出应用。",