feat(web): shared collection header, Play and Shuffle, and the centred page column (M494)

- CollectionHeader holds cover, title, facts, then Play and Shuffle, for
  album and playlist. The playlist page gains Play and Shuffle; it had
  neither. The cover uses the shared Cover component, so the playlist
  header also gets the fallback image.
- PlayActions is the one Play and Shuffle pair: filled Play, outlined
  Shuffle, on album, playlist and artist pages (operator, 2026-10-08).
  The artist page's round icon button said Play but shuffled; Play now
  keeps the server's order, and Shuffle shuffles.
- PageColumn is the playlist page's centred column, now used by album,
  playlist, Liked, History and Search → Tracks.
- The playlist page builds its queue with playlistTrackToRef, the shared
  converter, in place of its own copy. Its copy skipped only removed
  tracks, so a missing file (#2527) could still be queued from there.
- shuffled() in lib/utils is the one shuffle, replacing three private
  copies (album page, artist page, ArtistCard).
- The album's refetch error now uses text-error-fg. `text-oxblood` is
  not a colour in the theme, so it never rendered.

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

web/src/lib/components/ArtistCard.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+  import { shuffled } from '#lib/utils/shuffle.js';
   import type { ArtistRef, TrackRef } from '#lib/api/types.js';
   import { api } from '#lib/api/client.js';
   import { playQueue, enqueueTracks } from '#lib/player/store.svelte.js';
@@ -12,21 +13,12 @@
 
   let queueing = $state(false);
 
-  function shuffle<T>(items: T[]): T[] {
-    const arr = items.slice();
-    for (let i = arr.length - 1; i > 0; i--) {
-      const j = Math.floor(Math.random() * (i + 1));
-      [arr[i], arr[j]] = [arr[j], arr[i]];
-    }
-    return arr;
-  }
-
   async function onPlayClick(e: MouseEvent) {
     e.preventDefault();
     e.stopPropagation();
     const tracks = await api.get<TrackRef[]>(`/api/artists/${artist.id}/tracks`);
     if (tracks.length === 0) return;
-    playQueue(shuffle(tracks), 0);
+    playQueue(shuffled(tracks), 0);
   }
 
   async function onAddToQueue(e: MouseEvent) {