refactor(web/api): offsetGetNextPageParam helper; paging sites migrated (W4)

Final task of PR2 DRY pass. Extracts the verbatim
`offset + items.length >= total` math repeated across 8
createInfiniteQuery sites into a single Page<T>-shape helper.

The task spec described a bare-array helper signature
(lastPage: T[], pageSize-based stop), but no call site in this
repo uses that shape — every paged endpoint returns a Page<T>
envelope. The helper is named pageGetNextPageParam and matches
the actual canonical shape so the 8 copies could collapse.

Migrated:
- likes.ts: 3 sites (TrackRef, AlbumRef, ArtistRef)
- albums.ts: 1 site (AlbumRef)
- queries.ts: 4 sites (artists + 3 search facets)

history.ts left alone — uses has_more/total, different shape.

web/src/lib/api/albums.ts

@@ -1,6 +1,7 @@
 import { createInfiniteQuery } from '@tanstack/svelte-query';
 import { api } from './client';
 import { qk } from './queries';
+import { pageGetNextPageParam } from './paging';
 import type { AlbumRef, AlbumDetail, Page, TrackRef } from './types';
 
 export const ALBUM_PAGE_SIZE = 50;
@@ -14,10 +15,7 @@ export function createAlbumsAlphaInfiniteQuery() {
     queryKey: qk.albumsAlpha(),
     queryFn: ({ pageParam = 0 }) => listAlbumsAlpha(ALBUM_PAGE_SIZE, pageParam),
     initialPageParam: 0,
-    getNextPageParam: (last) => {
-      const loaded = last.offset + last.items.length;
-      return loaded >= last.total ? undefined : loaded;
-    }
+    getNextPageParam: pageGetNextPageParam<AlbumRef>()
   });
 }