feat(privacy): 为 OpenAI OAuth 账号添加前端手动设置隐私按钮

复用已有的 set-privacy API 端点，Handler 通过 platform 分发到
ForceOpenAIPrivacy / ForceAntigravityPrivacy，前端 AccountActionMenu
扩展隐私按钮支持 OpenAI OAuth 账号。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/admin/AccountsView.vue

@@ -1262,7 +1262,7 @@ const handleSetPrivacy = async (a: Account) => {
     appStore.showSuccess(t('common.success'))
   } catch (error: any) {
     console.error('Failed to set privacy:', error)
-    appStore.showError(error?.response?.data?.message || t('admin.accounts.privacyAntigravityFailed'))
+    appStore.showError(error?.response?.data?.message || t('admin.accounts.privacyFailed'))
   }
 }
 const handleDelete = (a: Account) => { deletingAcc.value = a; showDeleteDialog.value = true }