fix: return degraded UsageInfo instead of 500 for Antigravity usage errors

When the Antigravity quota API returns non-403 errors (e.g. 401
UNAUTHENTICATED), return a UsageInfo with an error field instead of
propagating the error as HTTP 500. This matches Antigravity Manager's
behavior of gracefully displaying errors.

backend/internal/service/account_usage_service.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"log/slog"
 	"math/rand/v2"
 	"net/http"
 	"strings"
@@ -191,6 +192,9 @@ type UsageInfo struct {
 	ForbiddenReason string `json:"forbidden_reason,omitempty"`
 	ForbiddenType   string `json:"forbidden_type,omitempty"` // "validation" / "violation" / "forbidden"
 	ValidationURL   string `json:"validation_url,omitempty"` // 验证/申诉链接
+
+	// 获取 usage 时的错误信息（降级返回，而非 500）
+	Error string `json:"error,omitempty"`
 }
 
 // ClaudeUsageResponse Anthropic API返回的usage结构
@@ -666,7 +670,20 @@ func (s *AccountUsageService) getAntigravityUsage(ctx context.Context, account *
 	// 3. 调用 API 获取额度
 	result, err := s.antigravityQuotaFetcher.FetchQuota(ctx, account, proxyURL)
 	if err != nil {
-		return nil, fmt.Errorf("fetch antigravity quota failed: %w", err)
+		// 降级返回带 error 字段的 UsageInfo，而非 500
+		now := time.Now()
+		errMsg := fmt.Sprintf("usage API error: %v", err)
+		slog.Warn("antigravity usage fetch failed, returning degraded response",
+			"account_id", account.ID, "error", err)
+		degraded := &UsageInfo{
+			UpdatedAt: &now,
+			Error:     errMsg,
+		}
+		s.cache.antigravityCache.Store(account.ID, &antigravityUsageCache{
+			usageInfo: degraded,
+			timestamp: time.Now(),
+		})
+		return degraded, nil
 	}
 
 	// 4. 缓存结果

frontend/src/components/account/AccountUsageCell.vue

@@ -36,6 +36,10 @@
 
       <!-- Usage data -->
       <div v-else-if="usageInfo" class="space-y-1">
+        <!-- API error (degraded response) -->
+        <div v-if="usageInfo.error" class="text-xs text-amber-600 dark:text-amber-400 truncate max-w-[200px]" :title="usageInfo.error">
+          {{ usageInfo.error }}
+        </div>
         <!-- 5h Window -->
         <UsageProgressBar
           v-if="usageInfo.five_hour"

frontend/src/types/index.ts

@@ -784,6 +784,7 @@ export interface AccountUsageInfo {
   forbidden_reason?: string
   forbidden_type?: string   // "validation" | "violation" | "forbidden"
   validation_url?: string   // 验证/申诉链接
+  error?: string            // usage 获取失败时的错误信息
 }
 
 // OpenAI Codex usage snapshot (from response headers)