chore(i18n): 补充界面缩放设置的中英文文案并清理多余键

- en-US / zh-CN 新增 app.appearance-settings.ui-scale.title 与
  .description；
- 移除未使用的 ui-scale.custom / ui-scale.percent 文案键。

apps/app-frontend/src/components/ui/settings/AppearanceSettings.vue

@@ -219,14 +219,6 @@ const messages = defineMessages({
 		defaultMessage:
 			'Scale the whole launcher interface. A smaller scale fits more content, which helps when embedded pages are cut off.',
 	},
-	uiScaleCustom: {
-		id: 'app.appearance-settings.ui-scale.custom',
-		defaultMessage: 'Custom',
-	},
-	uiScalePercent: {
-		id: 'app.appearance-settings.ui-scale.percent',
-		defaultMessage: '{value}%',
-	},
 	pageTransitionsTitle: {
 		id: 'app.appearance-settings.page-transitions.title',
 		defaultMessage: 'Page transition animations',

apps/app-frontend/src/locales/en-US/index.json

@@ -773,6 +773,12 @@
   "app.appearance-settings.transparent-background.title": {
     "message": "Transparent background"
   },
+  "app.appearance-settings.ui-scale.title": {
+    "message": "Launcher scale"
+  },
+  "app.appearance-settings.ui-scale.description": {
+    "message": "Scale the whole launcher interface. A smaller scale fits more content, which helps when embedded pages are cut off."
+  },
   "app.appearance-settings.unknown-pack-warning.description": {
     "message": "If you attempt to install a Modrinth Pack file (.mrpack) that isn't hosted on Modrinth, we'll make sure you understand the risks before installing it."
   },

apps/app-frontend/src/locales/zh-CN/index.json

@@ -805,6 +805,12 @@
   "app.appearance-settings.transparent-background.title": {
     "message": "透明背景"
   },
+  "app.appearance-settings.ui-scale.title": {
+    "message": "启动器缩放"
+  },
+  "app.appearance-settings.ui-scale.description": {
+    "message": "缩放整个启动器界面。缩小后能显示更多内容，适合内嵌网页被裁切的情况。"
+  },
   "app.appearance-settings.unknown-pack-warning.description": {
     "message": "如果你尝试安装一个未托管在 Modrinth 上的 Modrinth 整合包文件（.mrpack），我们会确保你在安装之前了解相关风险。"
   },