fix: throttle Anthropic usage queries and pass through upstream HTTP errors

- Frontend: queue Anthropic OAuth/setup-token usage requests by proxy
  with random 1-1.5s interval to prevent upstream 429
- Backend: return ApplicationError with actual upstream status code
  instead of wrapping all errors as 500
- Handle component unmount to skip stale updates on page navigation

backend/internal/repository/claude_usage_service.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"time"
 
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
@@ -95,7 +96,8 @@ func (s *claudeUsageService) FetchUsageWithOptions(ctx context.Context, opts *se
 
 	if resp.StatusCode != http.StatusOK {
 		body, _ := io.ReadAll(resp.Body)
-		return nil, fmt.Errorf("API returned status %d: %s", resp.StatusCode, string(body))
+		msg := fmt.Sprintf("API returned status %d: %s", resp.StatusCode, string(body))
+		return nil, infraerrors.New(resp.StatusCode, "UPSTREAM_ERROR", msg)
 	}
 
 	var usageResp service.ClaudeUsageResponse