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/i18n/locales/en.ts

@@ -1157,7 +1157,11 @@ export default {
         noData: 'No usage data available for this account'
       },
       usageWindow: {
-        statsTitle: '5-Hour Window Usage Statistics'
+        statsTitle: '5-Hour Window Usage Statistics',
+        gemini3Pro: 'G3P',
+        gemini3Flash: 'G3F',
+        gemini3Image: 'G3I',
+        claude45: 'C4.5'
       }
     },
 

frontend/src/i18n/locales/zh.ts

@@ -966,7 +966,11 @@ export default {
         cooldown: '冷却中'
       },
       usageWindow: {
-        statsTitle: '5小时窗口用量统计'
+        statsTitle: '5小时窗口用量统计',
+        gemini3Pro: 'G3P',
+        gemini3Flash: 'G3F',
+        gemini3Image: 'G3I',
+        claude45: 'C4.5'
       },
       form: {
         nameLabel: '账号名称',