feat: 添加主题管理功能，更新系统设置页面，优化用户体验

src/locales/en-US.json

@@ -315,6 +315,11 @@
     "updateNow": "Update Now",
     "alreadyLatest": "Already up to date",
     "checkUpdateFailed": "Failed to check for updates",
-    "languageTitle": "Language / 语言"
+    "languageTitle": "Language / 语言",
+    "theme": "Theme",
+    "themeTitle": "Appearance",
+    "themeLight": "Light",
+    "themeDark": "Dark",
+    "themeAuto": "Auto"
   }
 }