fix: 修复开发者功能开关页面空白

FeatureFlagSettings.vue 使用了 ref 但未从 vue 导入，导致
<script setup> 顶层抛出 ReferenceError，整页渲染失败（功能开关列表
空白）。补上 ref 的导入。

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

@@ -7,7 +7,7 @@ import {
 	Toggle,
 	useVIntl,
 } from '@modrinth/ui'
-import { inject, watch } from 'vue'
+import { inject, ref, watch } from 'vue'
 
 import { get as getSettings, set as setSettings } from '@/helpers/settings.ts'
 import { isDev } from '@/helpers/utils'