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.test.ts

@@ -566,6 +566,22 @@ describe('http client', () => {
     })
   })
 
+  it('returns null when a successful response carries null data', async () => {
+    const fetchMock = vi.mocked(fetch)
+    fetchMock.mockResolvedValueOnce(
+      jsonResponse({
+        code: 0,
+        message: 'ok',
+        data: null,
+      }),
+    )
+
+    const { get } = await loadModules()
+    const result = await get<null>('/nullable-success', undefined, { auth: false })
+
+    expect(result).toBeNull()
+  })
+
   it('converts aborted requests into timeout AppErrors', async () => {
     vi.useFakeTimers()
     const fetchMock = vi.mocked(fetch)

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')

frontend/admin/src/types/http.ts

@@ -11,7 +11,7 @@ export interface ApiResponse<T> {
   /** 响应消息 */
   message: string
   /** 响应数据 */
-  data: T
+  data: T | null
 }
 
 /**

frontend/admin/src/types/http.typecheck.ts

@@ -0,0 +1,7 @@
+import type { ApiResponse } from './http'
+
+export const nullableSuccessResponseContract: ApiResponse<{ ok: true }> = {
+  code: 0,
+  message: 'ok',
+  data: null,
+}