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/types/index.ts

@@ -769,6 +769,21 @@ export interface AccountUsageInfo {
   gemini_pro_minute?: UsageProgress | null
   gemini_flash_minute?: UsageProgress | null
   antigravity_quota?: Record<string, AntigravityModelQuota> | null
+  // Antigravity 403 forbidden 状态
+  is_forbidden?: boolean
+  forbidden_reason?: string
+  forbidden_type?: string   // "validation" | "violation" | "forbidden"
+  validation_url?: string   // 验证/申诉链接
+
+  // 状态标记（后端自动推导）
+  needs_verify?: boolean    // 需要人工验证（forbidden_type=validation）
+  is_banned?: boolean       // 账号被封（forbidden_type=violation）
+  needs_reauth?: boolean    // token 失效需重新授权（401）
+
+  // 机器可读错误码：forbidden / unauthenticated / rate_limited / network_error
+  error_code?: string
+
+  error?: string            // usage 获取失败时的错误信息
 }
 
 // OpenAI Codex usage snapshot (from response headers)