feat(gemini): 添加 Google One 存储空间推断 Tier 功能

## 功能概述
通过 Google Drive API 获取存储空间配额来推断 Google One 订阅等级，并优化统一的配额显示系统。

## 后端改动
- 新增 Drive API 客户端 (drive_client.go)
  - 支持代理和指数退避重试
  - 处理 403/429 错误
- 添加 Tier 推断逻辑 (inferGoogleOneTier)
  - 支持 6 种 tier 类型：AI_PREMIUM, GOOGLE_ONE_STANDARD, GOOGLE_ONE_BASIC, FREE, GOOGLE_ONE_UNKNOWN, GOOGLE_ONE_UNLIMITED
- 集成到 OAuth 流程
  - ExchangeCode: 授权时自动获取 tier
  - RefreshAccountToken: Token 刷新时更新 tier (24小时缓存)
- 新增管理 API 端点
  - POST /api/v1/admin/accounts/:id/refresh-tier (单个账号刷新)
  - POST /api/v1/admin/accounts/batch-refresh-tier (批量刷新)

## 前端改动
- 更新 AccountQuotaInfo.vue
  - 添加 Google One tier 标签映射
  - 添加 tier 颜色样式 (紫色/蓝色/绿色/灰色/琥珀色)
- 更新 AccountUsageCell.vue
  - 添加 Google One tier 显示逻辑
  - 根据 oauth_type 区分显示方式
- 添加国际化翻译 (en.ts, zh.ts)
  - aiPremium, standard, basic, free, personal, unlimited

## Tier 推断规则
- >= 2TB: AI Premium
- >= 200GB: Google One Standard
- >= 100GB: Google One Basic
- >= 15GB: Free
- > 100TB: Unlimited (G Suite legacy)
- 其他/失败: Unknown (显示为 Personal)

## 优雅降级
- Drive API 失败时使用 GOOGLE_ONE_UNKNOWN
- 不阻断 OAuth 流程
- 24小时缓存避免频繁调用

## 测试
- ✅ 后端编译成功
- ✅ 前端构建成功
- ✅ 所有代码符合现有规范

frontend/src/components/account/AccountUsageCell.vue

@@ -568,6 +568,24 @@ const isGeminiCodeAssist = computed(() => {
 // Gemini 账户类型显示标签
 const geminiTierLabel = computed(() => {
   if (!geminiTier.value) return null
+
+  const creds = props.account.credentials as GeminiCredentials | undefined
+  const isGoogleOne = creds?.oauth_type === 'google_one'
+
+  if (isGoogleOne) {
+    // Google One tier 标签
+    const tierMap: Record<string, string> = {
+      AI_PREMIUM: t('admin.accounts.tier.aiPremium'),
+      GOOGLE_ONE_STANDARD: t('admin.accounts.tier.standard'),
+      GOOGLE_ONE_BASIC: t('admin.accounts.tier.basic'),
+      FREE: t('admin.accounts.tier.free'),
+      GOOGLE_ONE_UNKNOWN: t('admin.accounts.tier.personal'),
+      GOOGLE_ONE_UNLIMITED: t('admin.accounts.tier.unlimited')
+    }
+    return tierMap[geminiTier.value] || t('admin.accounts.tier.personal')
+  }
+
+  // Code Assist tier 标签
   const tierMap: Record<string, string> = {
     LEGACY: t('admin.accounts.tier.free'),
     PRO: t('admin.accounts.tier.pro'),
@@ -578,6 +596,25 @@ const geminiTierLabel = computed(() => {
 
 // Gemini 账户类型徽章样式
 const geminiTierClass = computed(() => {
+  if (!geminiTier.value) return ''
+
+  const creds = props.account.credentials as GeminiCredentials | undefined
+  const isGoogleOne = creds?.oauth_type === 'google_one'
+
+  if (isGoogleOne) {
+    // Google One tier 颜色
+    const colorMap: Record<string, string> = {
+      AI_PREMIUM: 'bg-purple-100 text-purple-600 dark:bg-purple-900/40 dark:text-purple-300',
+      GOOGLE_ONE_STANDARD: 'bg-blue-100 text-blue-600 dark:bg-blue-900/40 dark:text-blue-300',
+      GOOGLE_ONE_BASIC: 'bg-green-100 text-green-600 dark:bg-green-900/40 dark:text-green-300',
+      FREE: 'bg-gray-100 text-gray-600 dark:bg-gray-700 dark:text-gray-300',
+      GOOGLE_ONE_UNKNOWN: 'bg-gray-100 text-gray-600 dark:bg-gray-700 dark:text-gray-300',
+      GOOGLE_ONE_UNLIMITED: 'bg-amber-100 text-amber-600 dark:bg-amber-900/40 dark:text-amber-300'
+    }
+    return colorMap[geminiTier.value] || 'bg-gray-100 text-gray-600 dark:bg-gray-700 dark:text-gray-300'
+  }
+
+  // Code Assist tier 颜色
   switch (geminiTier.value) {
     case 'LEGACY':
       return 'bg-gray-100 text-gray-600 dark:bg-gray-700 dark:text-gray-300'