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/TabStrip.svelte

@@ -0,0 +1,47 @@
+<script lang="ts" generics="Id extends string">
+  // In-page tabs: one strip of buttons switching what a single page shows
+  // (#5416). The same look as AdminTabs and DiscoverTabs, which are route
+  // navigation and so stay links; this is for state inside one page, such as
+  // Liked's Artists | Albums | Tracks or a queue's status filter.
+  type Item = {
+    id: Id;
+    label: string;
+    /** Shown as a pill after the label. Omit for no pill. */
+    count?: number;
+  };
+
+  let {
+    items,
+    value = $bindable(),
+    ariaLabel
+  }: {
+    items: Item[];
+    value: Id;
+    ariaLabel: string;
+  } = $props();
+</script>
+
+<div role="tablist" aria-label={ariaLabel} class="flex gap-2 border-b border-border">
+  {#each items as item (item.id)}
+    {@const active = value === item.id}
+    <button
+      type="button"
+      role="tab"
+      aria-selected={active}
+      class="border-b-2 px-3 py-2 text-sm transition-colors {active
+        ? 'border-accent text-text-primary'
+        : 'border-transparent text-text-secondary hover:text-text-primary'}"
+      onclick={() => (value = item.id)}
+    >
+      {item.label}
+      {#if item.count !== undefined}
+        <span
+          class="ml-1.5 inline-flex items-center rounded-full bg-accent-tint px-2 py-0.5 text-xs text-accent-fg"
+          data-testid="tab-count"
+        >
+          {item.count}
+        </span>
+      {/if}
+    </button>
+  {/each}
+</div>