feat(web): infinite scroll on /library/artists and /library/albums

Replaces the explicit 'Load more' button on both library list pages with
an IntersectionObserver-based sentinel that triggers fetchNextPage
automatically as the operator scrolls near the bottom (300px rootMargin).

New InfiniteScrollSentinel component:
- Stays disabled while a fetch is in flight (prevents repeat firing on
  tall pages or fast scrolls).
- Tears down its observer when hasNextPage flips false.
- Defensive against environments without IntersectionObserver (jsdom);
  tests provide a synchronous mock.

Also fixes a pre-existing test setup gap on the albums page test:
AlbumCard renders LikeButton which calls useQueryClient(), and the
page.test.ts wasn't wrapped in a QueryClientProvider — added the same
useQueryClient mock the AlbumCard.test.ts already uses.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/routes/library/albums/page.test.ts

@@ -37,6 +37,13 @@ vi.mock('$lib/api/likes', () => ({
   likeEntity: vi.fn(),
   unlikeEntity: vi.fn()
 }));
+// AlbumCard renders LikeButton, which calls useQueryClient() to get the
+// cache for invalidation. The page test isn't wrapped in a QueryClientProvider,
+// so we stub useQueryClient with a noop client.
+vi.mock('@tanstack/svelte-query', async (orig) => {
+  const actual = (await orig()) as Record<string, unknown>;
+  return { ...actual, useQueryClient: () => ({}) };
+});
 
 import Page from './+page.svelte';