test(settings): cover fill-crop preference sync payload

Guard against the display preference mismatch reported in #1290 by checking author fields in cover-mode saves and verifying server acceptance of the complete payload.

Ref #1290

client/src/composables/__tests__/useDisplaySettingsSync.spec.ts

@@ -150,9 +150,18 @@ describe('useDisplaySettingsSync', () => {
       '/api/v1/user-preferences/display',
       expect.objectContaining({
         method: 'PUT',
-        body: expect.stringContaining('"bookCoverDisplayMode":"fill-crop"'),
+        body: expect.any(String),
       }),
     )
+
+    const request = apiMock.mock.calls[0]?.[1] as RequestInit
+    expect(JSON.parse(request.body as string)).toMatchObject({
+      settings: {
+        authorRowDensity: 'comfortable',
+        authorCoverFallback: false,
+        bookCoverDisplayMode: 'fill-crop',
+      },
+    })
   })
 
   it('cancels a pending debounced save', async () => {