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/lib/components/InfiniteScrollSentinel.svelte

@@ -0,0 +1,35 @@
+<script lang="ts">
+  // Renders an invisible sentinel element near the bottom of a paged list.
+  // When the sentinel enters the viewport (within `rootMargin`), onIntersect
+  // fires — typically calling TanStack Query's fetchNextPage. The caller is
+  // responsible for setting `enabled = hasNextPage && !isFetchingNextPage`
+  // so the observer stays disconnected while a fetch is in flight; otherwise
+  // a tall list could fire onIntersect repeatedly during a single scroll.
+
+  let {
+    onIntersect,
+    enabled = true,
+    rootMargin = '300px'
+  }: {
+    onIntersect: () => void;
+    enabled?: boolean;
+    rootMargin?: string;
+  } = $props();
+
+  let sentinel: HTMLElement | undefined = $state();
+
+  $effect(() => {
+    if (!enabled || !sentinel) return;
+    if (typeof IntersectionObserver === 'undefined') return;
+    const observer = new IntersectionObserver(
+      (entries) => {
+        if (entries[0]?.isIntersecting) onIntersect();
+      },
+      { rootMargin }
+    );
+    observer.observe(sentinel);
+    return () => observer.disconnect();
+  });
+</script>
+
+<div bind:this={sentinel} aria-hidden="true" class="h-px"></div>