fix(sync): propagate group membership for already-synced books (#4946)

Group membership synced only for newly-imported books. Re-grouping a
book already present on both devices bumped book.updatedAt and won the
library-index LWW race, but mergeBookMetadata dropped groupId/groupName
from the overlay, so the change never reached peers. New books instead
arrive via addBookToLibrary with the full remote object, which is why
their group did travel.

Carry groupId/groupName in mergeBookMetadata, matching native cloud
sync (transform.ts maps group_id/group_name). Values are assigned raw
so a group removal also propagates.

Fixes #4942

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/readest-app/src/__tests__/services/sync/file/engine-metadata-sync.test.ts

@@ -130,6 +130,33 @@ describe('FileSyncEngine metadata reconciliation (#4756)', () => {
     expect(indexedBook.title).toBe('New Title');
   });
 
+  test('pulls newer remote group membership for a book the device already has (#4942)', async () => {
+    const local = makeLocalBook({ updatedAt: 100 });
+    const remote = makeLocalBook({ groupId: 'g1', groupName: 'Sci-Fi', updatedAt: 200 });
+    const capture: { index?: RemoteLibraryIndex | null } = {};
+    const provider = makeProvider(makeRemoteIndex(remote, 200), null, capture);
+
+    const updateBookMetadata = vi.fn(async (_book: Book) => {});
+    const store = makeStore({ updateBookMetadata });
+
+    const engine = new FileSyncEngine(provider, store);
+    const result = await engine.syncLibrary([local], {
+      strategy: 'silent',
+      syncBooks: false,
+      deviceId: 'pc-device',
+    });
+
+    expect(updateBookMetadata).toHaveBeenCalledTimes(1);
+    const merged = updateBookMetadata.mock.calls[0]![0];
+    expect(merged.groupId).toBe('g1');
+    expect(merged.groupName).toBe('Sci-Fi');
+    expect(result.metadataUpdated).toBe(1);
+
+    const indexedBook = capture.index!.books.find((b) => b.hash === 'h1')!;
+    expect(indexedBook.groupId).toBe('g1');
+    expect(indexedBook.groupName).toBe('Sci-Fi');
+  });
+
   test('does not overwrite local metadata when the local copy is newer', async () => {
     const local = makeLocalBook({ title: 'Local Newer', updatedAt: 300 });
     const remote = makeLocalBook({ title: 'Remote Older', updatedAt: 200 });

apps/readest-app/src/__tests__/services/sync/file/merge.test.ts

@@ -129,6 +129,36 @@ describe('mergeBookMetadata (LWW field subset)', () => {
     expect(m.progress).toEqual([1, 2]);
     expect(m.updatedAt).toBe(9);
   });
+
+  test('carries remote group membership (add-to-group) when remote is newer (#4942)', () => {
+    const local = { hash: 'h', title: 'T', author: 'A', updatedAt: 1 } as Book;
+    const remote = {
+      hash: 'h',
+      title: 'T',
+      author: 'A',
+      groupId: 'g1',
+      groupName: 'Sci-Fi',
+      updatedAt: 9,
+    } as Book;
+    const m = mergeBookMetadata(local, remote);
+    expect(m.groupId).toBe('g1');
+    expect(m.groupName).toBe('Sci-Fi');
+  });
+
+  test('propagates group removal when remote cleared membership (#4942)', () => {
+    const local = {
+      hash: 'h',
+      title: 'T',
+      author: 'A',
+      groupId: 'g1',
+      groupName: 'Sci-Fi',
+      updatedAt: 1,
+    } as Book;
+    const remote = { hash: 'h', title: 'T', author: 'A', updatedAt: 9 } as Book;
+    const m = mergeBookMetadata(local, remote);
+    expect(m.groupId).toBeUndefined();
+    expect(m.groupName).toBeUndefined();
+  });
 });
 
 describe('isRemoteBookMetadataNewer', () => {

apps/readest-app/src/services/sync/file/merge.ts

@@ -87,14 +87,21 @@ export const mergeBookConfig = (
  * Overlay the user-facing metadata of `remote` onto `local`, preserving every
  * device-local / file-system field: `filePath`, `sourceTitle` (which names the
  * on-disk file), `coverImageUrl` (a device-local blob URL the caller
- * regenerates), reading progress, reading status, group membership, `hash`,
- * `format`, `createdAt`, etc.
+ * regenerates), reading progress, reading status, `hash`, `format`,
+ * `createdAt`, etc.
  *
- * Only the fields a metadata edit actually changes travel — this list mirrors
- * `getBookWithUpdatedMetadata` in `utils/book.ts`, which is the local side of
- * the same operation. The cover image is replicated separately as cover.png
- * bytes (see the reconciliation pass in the engine), so it is intentionally
- * absent here.
+ * Group membership (`groupId` / `groupName`) is user-facing metadata and travels
+ * too, matching the native cloud sync (`transform.ts` maps `group_id` /
+ * `group_name`). Without this, re-grouping a book already present on both
+ * devices bumps `updatedAt` and wins LWW, yet the group change is dropped by the
+ * overlay — so it never propagated for already-synced books (#4942). Assigning
+ * the raw remote values (not `?? local`) lets a group removal (undefined) clear
+ * membership on peers too.
+ *
+ * The scalar fields a metadata edit changes mirror `getBookWithUpdatedMetadata`
+ * in `utils/book.ts`, which is the local side of the same operation. The cover
+ * image is replicated separately as cover.png bytes (see the reconciliation pass
+ * in the engine), so it is intentionally absent here.
  */
 export const mergeBookMetadata = (local: Book, remote: Book): Book => ({
   ...local,
@@ -102,6 +109,8 @@ export const mergeBookMetadata = (local: Book, remote: Book): Book => ({
   author: remote.author,
   metadata: remote.metadata ?? local.metadata,
   primaryLanguage: remote.primaryLanguage ?? local.primaryLanguage,
+  groupId: remote.groupId,
+  groupName: remote.groupName,
   updatedAt: remote.updatedAt,
 });