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/test-utils/intersectionObserver.ts

@@ -0,0 +1,40 @@
+import { vi } from 'vitest';
+
+// jsdom doesn't ship IntersectionObserver, so a list that loads as you scroll
+// (InfiniteScrollSentinel, ListContinuation) never fires in a test. This
+// installs a stand-in that records each observer, so a test can play
+// "the reader scrolled to the bottom" with scrollToEnd().
+type Observer = { cb: IntersectionObserverCallback; self: IntersectionObserver; connected: boolean };
+
+export function installIntersectionObserverMock() {
+  const observers: Observer[] = [];
+
+  class MockIntersectionObserver {
+    observe = vi.fn();
+    unobserve = vi.fn();
+    disconnect = vi.fn(() => {
+      const o = observers.find((x) => x.self === (this as unknown as IntersectionObserver));
+      if (o) o.connected = false;
+    });
+    takeRecords = vi.fn(() => []);
+    constructor(cb: IntersectionObserverCallback) {
+      observers.push({ cb, self: this as unknown as IntersectionObserver, connected: true });
+    }
+  }
+
+  vi.stubGlobal('IntersectionObserver', MockIntersectionObserver);
+
+  return {
+    /** Reports every connected observer's target as on screen. */
+    scrollToEnd() {
+      for (const o of observers) {
+        if (!o.connected) continue;
+        o.cb([{ isIntersecting: true } as IntersectionObserverEntry], o.self);
+      }
+    },
+    /** How many observers are currently watching. */
+    connectedCount() {
+      return observers.filter((o) => o.connected).length;
+    }
+  };
+}

web/src/test-utils/query.ts

@@ -14,6 +14,7 @@ export function mockInfiniteQuery<T>(opts: {
   error?: unknown;
   hasNextPage?: boolean;
   isFetchingNextPage?: boolean;
+  isFetchNextPageError?: boolean;
   fetchNextPage?: () => void;
   refetch?: () => void;
 } = {}) {
@@ -24,6 +25,7 @@ export function mockInfiniteQuery<T>(opts: {
     error: opts.error,
     hasNextPage: opts.hasNextPage ?? false,
     isFetchingNextPage: opts.isFetchingNextPage ?? false,
+    isFetchNextPageError: opts.isFetchNextPageError ?? false,
     fetchNextPage: opts.fetchNextPage ?? (() => {}),
     refetch: opts.refetch ?? (() => {})
   });