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/routes/library/history/+page.svelte

@@ -5,7 +5,7 @@
   import { groupByDay } from '#lib/utils/dayGroup.js';
   import HistoryRow from '#lib/components/HistoryRow.svelte';
   import TrackList from '#lib/components/TrackList.svelte';
-  import InfiniteScrollSentinel from '#lib/components/InfiniteScrollSentinel.svelte';
+  import ListContinuation from '#lib/components/ListContinuation.svelte';
   import LibrarySkeleton from '#lib/components/LibrarySkeleton.svelte';
   import ApiErrorBanner from '#lib/components/ApiErrorBanner.svelte';
   import QuickFilter from '#lib/components/QuickFilter.svelte';
@@ -40,7 +40,7 @@
 
   {#if query.isPending}
     <LibrarySkeleton variant="list" count={12} />
-  {:else if query.isError}
+  {:else if query.isError && !query.isFetchNextPageError}
     <ApiErrorBanner error={query.error} onRetry={query.refetch} />
   {:else if flatEvents.length === 0}
     <EmptyState
@@ -72,16 +72,12 @@
       </section>
     {/each}
 
-    {#if query.hasNextPage}
-      <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 flatEvents.length > 0}
-      <p class="py-2 text-center text-sm text-text-secondary">End of history</p>
-    {/if}
+    <ListContinuation
+      hasMore={!!query.hasNextPage}
+      loading={query.isFetchingNextPage}
+      failed={query.isFetchNextPageError}
+      onMore={() => query.fetchNextPage()}
+      endLabel="End of history"
+    />
   {/if}
 </PageColumn>