feat(web): every paged list loads as you scroll; Liked becomes tabs (#5416)

Preference 172: no "Load more" buttons. Eight remained, on Liked (three
sections), the three Search overflow pages, Genres and Years.

- ListContinuation: the one bottom-of-list block. It loads the next page
  ahead of the reader, announces "Loading more…" through aria-live, shows an
  optional end line, and on a failed page shows the error with Try again,
  dropping the sentinel so a dead endpoint isn't re-hit on every scroll.
  Used on all ten paged lists, including the four that already autoloaded.
- A failed next page no longer replaces the list. TanStack sets isError for
  it, so the page-level error branches now apply only to a failed first
  load. Genres and Years did the same with their own loader; a later
  page's failure now keeps the grid.
- Liked: Artists | Albums | Tracks tabs (operator's choice). Stacked, a
  long Artists list loading as you scroll would bury the other two. It
  opens on the first tab that has likes.
- TabStrip: the in-page tab strip, now shared by Liked, Playback errors
  and Requests.
- test-utils/intersectionObserver: a stand-in so tests can scroll to the end.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/components/ListContinuation.svelte

@@ -0,0 +1,49 @@
+<script lang="ts">
+  // The bottom of every paged list (#5416, preference 172): the next page
+  // loads on its own as the reader nears the end, with no "Load more"
+  // button. What the button used to give for free is kept here:
+  //
+  // - In flight: a "Loading more…" line, announced through aria-live, so the
+  //   list doesn't look finished while it isn't.
+  // - Finished: an optional quiet end line (`endLabel`).
+  // - Failed: the error and a Try again button. The sentinel is removed while
+  //   a page has failed, so a dead endpoint isn't re-requested on every scroll.
+  import InfiniteScrollSentinel from './InfiniteScrollSentinel.svelte';
+
+  let {
+    hasMore,
+    loading,
+    failed = false,
+    onMore,
+    endLabel
+  }: {
+    /** Whether another page exists. */
+    hasMore: boolean;
+    /** A next page is in flight. */
+    loading: boolean;
+    /** The last next-page fetch failed. */
+    failed?: boolean;
+    /** Fetch the next page (TanStack's fetchNextPage, or a page's own loader). */
+    onMore: () => void;
+    /** Shown once the list is complete, e.g. "End of history". */
+    endLabel?: string;
+  } = $props();
+</script>
+
+{#if failed}
+  <div role="alert" class="flex items-center justify-center gap-2 py-2 text-sm text-error-fg">
+    Couldn't load more.
+    <button
+      type="button"
+      class="rounded border border-border px-2 py-0.5 text-text-primary hover:bg-surface-hover
+             focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-accent"
+      onclick={onMore}>Try again</button
+    >
+  </div>
+{:else if hasMore}
+  <InfiniteScrollSentinel enabled={!loading} onIntersect={onMore} />
+{/if}
+
+<p class="py-2 text-center text-sm text-text-secondary empty:hidden" aria-live="polite">
+  {#if !failed && hasMore && loading}Loading more…{:else if !hasMore && endLabel}{endLabel}{/if}
+</p>