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.test.ts

@@ -0,0 +1,25 @@
+import { describe, expect, test } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/svelte';
+import TabStrip from './TabStrip.svelte';
+
+const items = [
+  { id: 'artists', label: 'Artists', count: 3 },
+  { id: 'albums', label: 'Albums' }
+];
+
+describe('TabStrip', () => {
+  test('a tablist with the selected tab marked, and counts where given', () => {
+    render(TabStrip, { props: { items, value: 'artists', ariaLabel: 'Liked' } });
+    expect(screen.getByRole('tablist', { name: 'Liked' })).toBeInTheDocument();
+    expect(screen.getByRole('tab', { name: /artists/i })).toHaveAttribute('aria-selected', 'true');
+    expect(screen.getByRole('tab', { name: /albums/i })).toHaveAttribute('aria-selected', 'false');
+    expect(screen.getAllByTestId('tab-count').map((c) => c.textContent?.trim())).toEqual(['3']);
+  });
+
+  test('clicking a tab selects it', async () => {
+    render(TabStrip, { props: { items, value: 'artists', ariaLabel: 'Liked' } });
+    await fireEvent.click(screen.getByRole('tab', { name: /albums/i }));
+    expect(screen.getByRole('tab', { name: /albums/i })).toHaveAttribute('aria-selected', 'true');
+    expect(screen.getByRole('tab', { name: /artists/i })).toHaveAttribute('aria-selected', 'false');
+  });
+});