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)