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/PlayActions.svelte

@@ -0,0 +1,53 @@
+<script lang="ts">
+  // Play and Shuffle, the same on every page that has them: album,
+  // playlist and artist (milestone 494). Play is filled and Shuffle
+  // outlined. The fill is the accent: the operator's call on 2026-10-08,
+  // made with the house rule against accent on action buttons stated to
+  // them.
+  import type { Snippet } from 'svelte';
+  import { Play, Shuffle } from 'lucide-svelte';
+
+  let {
+    name,
+    disabled = false,
+    onPlay,
+    onShuffle,
+    children
+  }: {
+    /** What is played, for the buttons' accessible names. */
+    name: string;
+    disabled?: boolean;
+    onPlay: () => void;
+    onShuffle: () => void;
+    /** Further actions after Shuffle. */
+    children?: Snippet;
+  } = $props();
+</script>
+
+<div class="flex flex-wrap items-center gap-2">
+  <button
+    type="button"
+    aria-label={`Play ${name}`}
+    onclick={onPlay}
+    {disabled}
+    class="flex items-center gap-2 rounded-md bg-accent px-4 py-2 text-sm font-medium
+           text-text-primary hover:opacity-90 focus-visible:ring-2 focus-visible:ring-accent
+           disabled:opacity-50"
+  >
+    <Play size={16} strokeWidth={1.5} fill="currentColor" />
+    Play
+  </button>
+  <button
+    type="button"
+    aria-label={`Shuffle ${name}`}
+    onclick={onShuffle}
+    {disabled}
+    class="flex items-center gap-2 rounded-md border border-border bg-surface px-4 py-2
+           text-sm font-medium text-text-primary hover:border-accent
+           focus-visible:ring-2 focus-visible:ring-accent disabled:opacity-50"
+  >
+    <Shuffle size={16} strokeWidth={1.5} />
+    Shuffle
+  </button>
+  {#if children}{@render children()}{/if}
+</div>