fix(gemini): 修复 P0/P1 级别问题（429误判/Tier丢失/expires_at/前端一致性）

P0 修复（Critical - 影响生产稳定性）:
- 修复 429 判断逻辑：使用 project_id 判断而非 account.Type
  防止 AI Studio OAuth 被误判为 Code Assist 5分钟窗口
- 修复 Tier ID 丢失：刷新时始终保留旧值，默认 LEGACY
  防止 fetchProjectID 失败导致 tier_id 被清空
- 修复 expires_at 下界：添加 minTTL=30s 保护
  防止 expires_in <= 300 时生成过去时间引发刷新风暴

P1 修复（Important - 行为一致性）:
- 前端 isCodeAssist 判断与后端一致（支持 legacy）
- 前端日期解析添加 NaN 保护
- 迁移脚本覆盖 legacy 账号

前端功能（新增）:
- AccountQuotaInfo 组件：Tier Badge + 二元进度条 + 倒计时
- 定时器动态管理：watch 监听限流状态
- 类型定义：GeminiCredentials 接口

测试:
- ✅ TypeScript 类型检查通过
- ✅ 前端构建成功（3.33s）
- ✅ Gemini + Codex 双 AI 审查通过

Refs: #gemini-quota

frontend/src/components/account/AccountQuotaInfo.vue

@@ -0,0 +1,194 @@
+<template>
+  <div v-if="shouldShowQuota" class="flex items-center gap-2">
+    <!-- Tier Badge -->
+    <span :class="['badge text-xs px-2 py-0.5 rounded font-medium', tierBadgeClass]">
+      {{ tierLabel }}
+    </span>
+
+    <!-- 限额文本 -->
+    <span class="text-xs text-gray-500 dark:text-gray-400">{{ quotaText }}</span>
+
+    <!-- 二元进度条 -->
+    <div class="group/progress relative flex items-center gap-1">
+      <div class="h-2 w-20 overflow-hidden rounded-full bg-gray-200 dark:bg-gray-700">
+        <div
+          :class="[
+            'h-full transition-all',
+            isRateLimited ? 'bg-red-500' : 'bg-gray-300 dark:bg-gray-600'
+          ]"
+          :style="{ width: isRateLimited ? '100%' : '0%' }"
+        ></div>
+      </div>
+
+      <span
+        :class="[
+          'text-xs font-medium',
+          isRateLimited ? 'text-red-600 dark:text-red-400' : 'text-gray-400 dark:text-gray-500'
+        ]"
+      >
+        {{ isRateLimited ? '100%' : '0%' }}
+      </span>
+
+      <!-- 倒计时 -->
+      <span
+        v-if="isRateLimited"
+        :class="[
+          'text-xs font-medium',
+          isUrgent
+            ? 'text-red-600 dark:text-red-400 animate-pulse'
+            : 'text-amber-600 dark:text-amber-400'
+        ]"
+      >
+        ⚠️ {{ resetCountdown }}
+      </span>
+
+      <!-- Tooltip -->
+      <div
+        class="pointer-events-none absolute -top-10 left-0 z-10 hidden whitespace-nowrap rounded bg-gray-900 px-2 py-1 text-xs text-white shadow-lg group-hover/progress:block dark:bg-gray-700"
+      >
+        ⓘ 无法提供实时额度
+        <div
+          class="absolute left-4 top-full border-4 border-transparent border-t-gray-900 dark:border-t-gray-700"
+        ></div>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed, ref, watch, onUnmounted } from 'vue'
+import type { Account, GeminiCredentials } from '@/types'
+
+const props = defineProps<{
+  account: Account
+}>()
+
+const now = ref(new Date())
+let timer: ReturnType<typeof setInterval> | null = null
+
+// 是否为 Code Assist OAuth
+// 判断逻辑与后端保持一致：project_id 存在即为 Code Assist
+const isCodeAssist = computed(() => {
+  const creds = props.account.credentials as GeminiCredentials | undefined
+  // 显式为 code_assist，或 legacy 情况（oauth_type 为空但 project_id 存在）
+  return creds?.oauth_type === 'code_assist' || (!creds?.oauth_type && !!creds?.project_id)
+})
+
+// 是否应该显示配额信息
+const shouldShowQuota = computed(() => {
+  return props.account.platform === 'gemini'
+})
+
+// Tier 标签文本
+const tierLabel = computed(() => {
+  if (isCodeAssist.value) {
+    const creds = props.account.credentials as GeminiCredentials | undefined
+    const tierMap: Record<string, string> = {
+      LEGACY: 'Free',
+      PRO: 'Standard',
+      ULTRA: 'Enterprise'
+    }
+    return tierMap[creds?.tier_id || ''] || 'Unknown'
+  }
+  return 'Gemini'
+})
+
+// Tier Badge 样式
+const tierBadgeClass = computed(() => {
+  if (!isCodeAssist.value) {
+    return 'bg-blue-100 text-blue-700 dark:bg-blue-900/30 dark:text-blue-400'
+  }
+  const creds = props.account.credentials as GeminiCredentials | undefined
+  const tierColorMap: Record<string, string> = {
+    LEGACY: 'bg-gray-100 text-gray-700 dark:bg-gray-900/30 dark:text-gray-400',
+    PRO: 'bg-blue-100 text-blue-700 dark:bg-blue-900/30 dark:text-blue-400',
+    ULTRA: 'bg-amber-100 text-amber-700 dark:bg-amber-900/30 dark:text-amber-400'
+  }
+  return (
+    tierColorMap[creds?.tier_id || ''] ||
+    'bg-gray-100 text-gray-700 dark:bg-gray-900/30 dark:text-gray-400'
+  )
+})
+
+// 限额文本
+const quotaText = computed(() => {
+  if (isCodeAssist.value) {
+    const creds = props.account.credentials as GeminiCredentials | undefined
+    const limitMap: Record<string, string> = {
+      LEGACY: '1000/day, 60/min',
+      PRO: '1500/day, 120/min',
+      ULTRA: '5000/day, 300/min'
+    }
+    return limitMap[creds?.tier_id || ''] || '-'
+  }
+  return 'RPM/RPD limits'
+})
+
+// 是否限流
+const isRateLimited = computed(() => {
+  if (!props.account.rate_limit_reset_at) return false
+  const resetTime = Date.parse(props.account.rate_limit_reset_at)
+  // 防护：如果日期解析失败（NaN），则认为未限流
+  if (Number.isNaN(resetTime)) return false
+  return resetTime > now.value.getTime()
+})
+
+// 倒计时文本
+const resetCountdown = computed(() => {
+  if (!props.account.rate_limit_reset_at) return ''
+  const resetTime = Date.parse(props.account.rate_limit_reset_at)
+  // 防护：如果日期解析失败，显示 "-"
+  if (Number.isNaN(resetTime)) return '-'
+
+  const diffMs = resetTime - now.value.getTime()
+  if (diffMs <= 0) return 'now'
+
+  const diffSeconds = Math.floor(diffMs / 1000)
+  const diffMinutes = Math.floor(diffSeconds / 60)
+  const diffHours = Math.floor(diffMinutes / 60)
+
+  if (diffMinutes < 1) return `${diffSeconds}s`
+  if (diffHours < 1) {
+    const secs = diffSeconds % 60
+    return `${diffMinutes}m ${secs}s`
+  }
+  const mins = diffMinutes % 60
+  return `${diffHours}h ${mins}m`
+})
+
+// 是否紧急（< 1分钟）
+const isUrgent = computed(() => {
+  if (!props.account.rate_limit_reset_at) return false
+  const resetTime = Date.parse(props.account.rate_limit_reset_at)
+  // 防护：如果日期解析失败，返回 false
+  if (Number.isNaN(resetTime)) return false
+
+  const diffMs = resetTime - now.value.getTime()
+  return diffMs > 0 && diffMs < 60000
+})
+
+// 监听限流状态，动态启动/停止定时器
+watch(
+  () => isRateLimited.value,
+  (limited) => {
+    if (limited && !timer) {
+      // 进入限流状态，启动定时器
+      timer = setInterval(() => {
+        now.value = new Date()
+      }, 1000)
+    } else if (!limited && timer) {
+      // 解除限流，停止定时器
+      clearInterval(timer)
+      timer = null
+    }
+  },
+  { immediate: true } // 立即执行，确保挂载时已限流的情况也能启动定时器
+)
+
+onUnmounted(() => {
+  if (timer !== null) {
+    clearInterval(timer)
+    timer = null
+  }
+})
+</script>

frontend/src/components/account/AccountUsageCell.vue

@@ -169,6 +169,11 @@
       <div v-else class="text-xs text-gray-400">-</div>
     </template>
 
+    <!-- Gemini platform: show quota info with AccountQuotaInfo component -->
+    <template v-else-if="account.platform === 'gemini'">
+      <AccountQuotaInfo :account="account" />
+    </template>
+
     <!-- Other accounts: no usage window -->
     <template v-else>
       <div class="text-xs text-gray-400">-</div>
@@ -176,7 +181,11 @@
   </div>
 
   <!-- Non-OAuth/Setup-Token accounts -->
-  <div v-else class="text-xs text-gray-400">-</div>
+  <div v-else>
+    <!-- Gemini API Key accounts: show quota info -->
+    <AccountQuotaInfo v-if="account.platform === 'gemini'" :account="account" />
+    <div v-else class="text-xs text-gray-400">-</div>
+  </div>
 </template>
 
 <script setup lang="ts">
@@ -185,6 +194,7 @@ import { useI18n } from 'vue-i18n'
 import { adminAPI } from '@/api/admin'
 import type { Account, AccountUsageInfo } from '@/types'
 import UsageProgressBar from './UsageProgressBar.vue'
+import AccountQuotaInfo from './AccountQuotaInfo.vue'
 
 const props = defineProps<{
   account: Account