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

backend/internal/pkg/antigravity/client.go

@@ -19,6 +19,16 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/proxyutil"
 )
 
+// ForbiddenError 表示上游返回 403 Forbidden
+type ForbiddenError struct {
+	StatusCode int
+	Body       string
+}
+
+func (e *ForbiddenError) Error() string {
+	return fmt.Sprintf("fetchAvailableModels 失败 (HTTP %d): %s", e.StatusCode, e.Body)
+}
+
 // NewAPIRequestWithURL 使用指定的 base URL 创建 Antigravity API 请求（v1internal 端点）
 func NewAPIRequestWithURL(ctx context.Context, baseURL, action, accessToken string, body []byte) (*http.Request, error) {
 	// 构建 URL，流式请求添加 ?alt=sse 参数
@@ -514,7 +524,20 @@ type ModelQuotaInfo struct {
 
 // ModelInfo 模型信息
 type ModelInfo struct {
-	QuotaInfo *ModelQuotaInfo `json:"quotaInfo,omitempty"`
+	QuotaInfo          *ModelQuotaInfo `json:"quotaInfo,omitempty"`
+	DisplayName        string          `json:"displayName,omitempty"`
+	SupportsImages     *bool           `json:"supportsImages,omitempty"`
+	SupportsThinking   *bool           `json:"supportsThinking,omitempty"`
+	ThinkingBudget     *int            `json:"thinkingBudget,omitempty"`
+	Recommended        *bool           `json:"recommended,omitempty"`
+	MaxTokens          *int            `json:"maxTokens,omitempty"`
+	MaxOutputTokens    *int            `json:"maxOutputTokens,omitempty"`
+	SupportedMimeTypes map[string]bool `json:"supportedMimeTypes,omitempty"`
+}
+
+// DeprecatedModelInfo 废弃模型转发信息
+type DeprecatedModelInfo struct {
+	NewModelID string `json:"newModelId"`
 }
 
 // FetchAvailableModelsRequest fetchAvailableModels 请求
@@ -524,7 +547,8 @@ type FetchAvailableModelsRequest struct {
 
 // FetchAvailableModelsResponse fetchAvailableModels 响应
 type FetchAvailableModelsResponse struct {
-	Models map[string]ModelInfo `json:"models"`
+	Models             map[string]ModelInfo           `json:"models"`
+	DeprecatedModelIDs map[string]DeprecatedModelInfo `json:"deprecatedModelIds,omitempty"`
 }
 
 // FetchAvailableModels 获取可用模型和配额信息，返回解析后的结构体和原始 JSON
@@ -573,6 +597,13 @@ func (c *Client) FetchAvailableModels(ctx context.Context, accessToken, projectI
 			continue
 		}
 
+		if resp.StatusCode == http.StatusForbidden {
+			return nil, nil, &ForbiddenError{
+				StatusCode: resp.StatusCode,
+				Body:       string(respBodyBytes),
+			}
+		}
+
 		if resp.StatusCode != http.StatusOK {
 			return nil, nil, fmt.Errorf("fetchAvailableModels 失败 (HTTP %d): %s", resp.StatusCode, string(respBodyBytes))
 		}