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/queries.ts

@@ -1,6 +1,15 @@
 import { createQuery, createInfiniteQuery } from '@tanstack/svelte-query';
 import { api } from './client';
-import type { ArtistRef, ArtistDetail, AlbumDetail, Page, SearchResponse } from './types';
+import { pageGetNextPageParam } from './paging';
+import type {
+  ArtistRef,
+  ArtistDetail,
+  AlbumRef,
+  AlbumDetail,
+  Page,
+  SearchResponse,
+  TrackRef
+} from './types';
 
 export type ArtistSort = 'alpha' | 'newest';
 export const ARTIST_PAGE_SIZE = 50;
@@ -60,10 +69,7 @@ export function createArtistsQuery(sort: ArtistSort) {
         `/api/artists?sort=${sort}&limit=${ARTIST_PAGE_SIZE}&offset=${pageParam}`
       ),
     initialPageParam: 0,
-    getNextPageParam: (last) => {
-      const loaded = last.offset + last.items.length;
-      return loaded >= last.total ? undefined : loaded;
-    },
+    getNextPageParam: pageGetNextPageParam<ArtistRef>(),
   });
 }
 
@@ -100,10 +106,7 @@ export function createSearchArtistsInfiniteQuery(q: string) {
         `/api/search?q=${encodeURIComponent(q)}&limit=${SEARCH_FACET_PAGE_SIZE}&offset=${pageParam}`
       ).then((r) => r.artists),
     initialPageParam: 0,
-    getNextPageParam: (last) => {
-      const loaded = last.offset + last.items.length;
-      return loaded >= last.total ? undefined : loaded;
-    },
+    getNextPageParam: pageGetNextPageParam<ArtistRef>(),
     enabled: q.length > 0
   });
 }
@@ -116,10 +119,7 @@ export function createSearchAlbumsInfiniteQuery(q: string) {
         `/api/search?q=${encodeURIComponent(q)}&limit=${SEARCH_FACET_PAGE_SIZE}&offset=${pageParam}`
       ).then((r) => r.albums),
     initialPageParam: 0,
-    getNextPageParam: (last) => {
-      const loaded = last.offset + last.items.length;
-      return loaded >= last.total ? undefined : loaded;
-    },
+    getNextPageParam: pageGetNextPageParam<AlbumRef>(),
     enabled: q.length > 0
   });
 }
@@ -132,10 +132,7 @@ export function createSearchTracksInfiniteQuery(q: string) {
         `/api/search?q=${encodeURIComponent(q)}&limit=${SEARCH_FACET_PAGE_SIZE}&offset=${pageParam}`
       ).then((r) => r.tracks),
     initialPageParam: 0,
-    getNextPageParam: (last) => {
-      const loaded = last.offset + last.items.length;
-      return loaded >= last.total ? undefined : loaded;
-    },
+    getNextPageParam: pageGetNextPageParam<TrackRef>(),
     enabled: q.length > 0
   });
 }