feat(antigravity): add 403 forbidden status detection, classification and display

Backend:
- Detect and classify 403 responses into three types:
  validation (account needs Google verification),
  violation (terms of service / banned),
  forbidden (generic 403)
- Extract verification/appeal URLs from 403 response body
  (structured JSON parsing with regex fallback)
- Add needs_verify, is_banned, needs_reauth, error_code fields
  to UsageInfo (omitempty for zero impact on other platforms)
- Handle 403 in request path: classify and permanently set account error
- Save validation_url in error_message for degraded path recovery
- Enrich usage with account error on both success and degraded paths
- Add singleflight dedup for usage requests with independent context
- Differentiate cache TTL: success/403 → 3min, errors → 1min
- Return degraded UsageInfo instead of HTTP 500 on quota fetch errors

Frontend:
- Display forbidden status badges with color coding (red for banned,
  amber for needs verification, gray for generic)
- Show clickable verification/appeal URL links
- Display needs_reauth and degraded error states in usage cell
- Add Antigravity tier label badge next to platform type

Tests:
- Comprehensive unit tests for classifyForbiddenType (7 cases)
- Unit tests for extractValidationURL (8 cases including unicode escapes)
- Integration test for FetchQuota forbidden path

frontend/src/components/layout/TablePageLayout.vue

@@ -84,9 +84,7 @@ onUnmounted(() => {
 }
 
 .table-scroll-container :deep(th) {
-  /* 表头高度和文字加粗优化 */
-  @apply px-5 py-4 text-left text-sm font-bold text-gray-900 dark:text-white border-b border-gray-200 dark:border-dark-700;
-  @apply uppercase tracking-wider; /* 让表头更有设计感 */
+  @apply px-5 py-4 text-left text-sm font-medium text-gray-600 dark:text-dark-300 border-b border-gray-200 dark:border-dark-700;
 }
 
 .table-scroll-container :deep(td) {