fix(frontend): ApiResponse data nullability contract

- Change ApiResponse.data from T to T | null to match backend reality
- Add compile-time type contract file (http.typecheck.ts)
- Maintain backward compatibility with existing service calls
- Add test for success response with null data

Refs: review-fix-closure-2026-05-28 ApiResponse nullability

frontend/admin/src/lib/http/client.ts

@@ -143,7 +143,7 @@ async function refreshAccessToken(): Promise<TokenBundle> {
     return cleanupSessionOnAuthFailure()
   }
 
-  return result.data
+  return result.data as TokenBundle
 }
 
 async function performTokenRefresh(): Promise<TokenBundle> {
@@ -293,7 +293,7 @@ async function request<T>(path: string, options: RequestOptions = {}): Promise<T
       throw AppError.fromResponse(result, response.status)
     }
 
-    return result.data
+    return result.data!
   } catch (error) {
     if (error instanceof DOMException && error.name === 'AbortError') {
       throw AppError.network('请求超时，请稍后重试')