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>

web/src/lib/components/InfiniteScrollSentinel.test.ts

@@ -0,0 +1,78 @@
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { render } from '@testing-library/svelte';
+import InfiniteScrollSentinel from './InfiniteScrollSentinel.svelte';
+
+// jsdom doesn't ship IntersectionObserver. We install a minimal mock that
+// captures the registered callback so tests can drive intersection events
+// synchronously.
+type IOInstance = {
+  observe: ReturnType<typeof vi.fn>;
+  disconnect: ReturnType<typeof vi.fn>;
+  fire: (isIntersecting: boolean) => void;
+};
+const created: IOInstance[] = [];
+
+beforeEach(() => {
+  created.length = 0;
+  (globalThis as unknown as { IntersectionObserver: unknown }).IntersectionObserver =
+    class MockIO {
+      private cb: IntersectionObserverCallback;
+      observe = vi.fn();
+      disconnect = vi.fn();
+      constructor(cb: IntersectionObserverCallback) {
+        this.cb = cb;
+        created.push({
+          observe: this.observe,
+          disconnect: this.disconnect,
+          fire: (isIntersecting: boolean) => {
+            this.cb(
+              [{ isIntersecting } as unknown as IntersectionObserverEntry],
+              this as unknown as IntersectionObserver
+            );
+          }
+        });
+      }
+    };
+});
+
+afterEach(() => {
+  vi.clearAllMocks();
+});
+
+describe('InfiniteScrollSentinel', () => {
+  test('observes the sentinel when enabled', () => {
+    const onIntersect = vi.fn();
+    render(InfiniteScrollSentinel, { props: { onIntersect, enabled: true } });
+    expect(created).toHaveLength(1);
+    expect(created[0].observe).toHaveBeenCalledTimes(1);
+  });
+
+  test('does not create an observer when enabled is false', () => {
+    const onIntersect = vi.fn();
+    render(InfiniteScrollSentinel, { props: { onIntersect, enabled: false } });
+    expect(created).toHaveLength(0);
+  });
+
+  test('fires onIntersect when the sentinel enters view', () => {
+    const onIntersect = vi.fn();
+    render(InfiniteScrollSentinel, { props: { onIntersect, enabled: true } });
+    created[0].fire(true);
+    expect(onIntersect).toHaveBeenCalledTimes(1);
+  });
+
+  test('does not fire onIntersect when the entry is not intersecting', () => {
+    const onIntersect = vi.fn();
+    render(InfiniteScrollSentinel, { props: { onIntersect, enabled: true } });
+    created[0].fire(false);
+    expect(onIntersect).not.toHaveBeenCalled();
+  });
+
+  test('disconnects the observer when the component unmounts', () => {
+    const onIntersect = vi.fn();
+    const { unmount } = render(InfiniteScrollSentinel, {
+      props: { onIntersect, enabled: true }
+    });
+    unmount();
+    expect(created[0].disconnect).toHaveBeenCalledTimes(1);
+  });
+});

web/src/routes/library/albums/+page.svelte

@@ -3,6 +3,7 @@
   import AlbumCard from '$lib/components/AlbumCard.svelte';
   import AlphabeticalGrid from '$lib/components/AlphabeticalGrid.svelte';
   import ApiErrorBanner from '$lib/components/ApiErrorBanner.svelte';
+  import InfiniteScrollSentinel from '$lib/components/InfiniteScrollSentinel.svelte';
   import { useDelayed } from '$lib/utils/useDelayed.svelte';
   import type { AlbumRef } from '$lib/api/types';
 
@@ -40,14 +41,13 @@
     </AlphabeticalGrid>
 
     {#if query.hasNextPage}
-      <button
-        type="button"
-        class="w-full rounded bg-surface py-2 text-sm hover:bg-surface-hover disabled:opacity-60"
-        disabled={query.isFetchingNextPage}
-        onclick={() => query.fetchNextPage()}
-      >
-        {query.isFetchingNextPage ? 'Loading…' : 'Load more'}
-      </button>
+      <InfiniteScrollSentinel
+        enabled={!query.isFetchingNextPage}
+        onIntersect={() => query.fetchNextPage()}
+      />
+      {#if query.isFetchingNextPage}
+        <p class="py-2 text-center text-sm text-text-secondary">Loading more…</p>
+      {/if}
     {:else if albums.length > 0}
       <p class="py-2 text-center text-sm text-text-secondary">End of library</p>
     {/if}

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';
 

web/src/routes/library/artists/+page.svelte

@@ -3,6 +3,7 @@
   import ArtistCard from '$lib/components/ArtistCard.svelte';
   import AlphabeticalGrid from '$lib/components/AlphabeticalGrid.svelte';
   import ApiErrorBanner from '$lib/components/ApiErrorBanner.svelte';
+  import InfiniteScrollSentinel from '$lib/components/InfiniteScrollSentinel.svelte';
   import { useDelayed } from '$lib/utils/useDelayed.svelte';
   import type { ArtistRef } from '$lib/api/types';
 
@@ -40,14 +41,13 @@
     </AlphabeticalGrid>
 
     {#if query.hasNextPage}
-      <button
-        type="button"
-        class="w-full rounded bg-surface py-2 text-sm hover:bg-surface-hover disabled:opacity-60"
-        disabled={query.isFetchingNextPage}
-        onclick={() => query.fetchNextPage()}
-      >
-        {query.isFetchingNextPage ? 'Loading…' : 'Load more'}
-      </button>
+      <InfiniteScrollSentinel
+        enabled={!query.isFetchingNextPage}
+        onIntersect={() => query.fetchNextPage()}
+      />
+      {#if query.isFetchingNextPage}
+        <p class="py-2 text-center text-sm text-text-secondary">Loading more…</p>
+      {/if}
     {:else if artists.length > 0}
       <p class="py-2 text-center text-sm text-text-secondary">End of library</p>
     {/if}