feat(antigravity): 添加配额窗口显示功能

后端:
- 新增 AntigravityQuotaRefresher 定时刷新配额
- Client 添加 FetchAvailableModels 方法获取模型配额
- 配额数据存入 account.extra.quota 字段

前端:
- AccountUsageCell 支持显示 Antigravity 账户配额
- UsageProgressBar 新增 amber 颜色
- 显示 G3P/G3F/G3I/C4.5 四个配额进度条

frontend/src/components/account/UsageProgressBar.vue

@@ -58,7 +58,7 @@ const props = defineProps<{
   label: string
   utilization: number // Percentage (0-100+)
   resetsAt?: string | null
-  color: 'indigo' | 'emerald' | 'purple'
+  color: 'indigo' | 'emerald' | 'purple' | 'amber'
   windowStats?: WindowStats | null
 }>()
 
@@ -69,7 +69,8 @@ const labelClass = computed(() => {
   const colors = {
     indigo: 'bg-indigo-100 text-indigo-700 dark:bg-indigo-900/40 dark:text-indigo-300',
     emerald: 'bg-emerald-100 text-emerald-700 dark:bg-emerald-900/40 dark:text-emerald-300',
-    purple: 'bg-purple-100 text-purple-700 dark:bg-purple-900/40 dark:text-purple-300'
+    purple: 'bg-purple-100 text-purple-700 dark:bg-purple-900/40 dark:text-purple-300',
+    amber: 'bg-amber-100 text-amber-700 dark:bg-amber-900/40 dark:text-amber-300'
   }
   return colors[props.color]
 })