fix(clients): never leave cover tiles blank — shared web <Cover> + Android Coil placeholder/error

Cover tiles (worst in the "You might like" home row, which surfaces
unplayed items whose art is often not yet backfilled) sat empty while
loading and stayed blank on a 404. The server returns a fast 404; the
gap was missing client-side loading/fallback states.

Web: new shared Cover.svelte owns the loading placeholder + onerror
fallback (static cover, or Disc3 for artists). AlbumCard, ArtistCard and
CompactTrackCard now reuse it instead of three hand-rolled <img> tags
that disagreed on fallback handling — notably ArtistCard had no onerror.

Android: ServerImage tracks Coil's load state so the per-caller fallback
doubles as a placeholder (loading) and an error state (404 / unreachable),
instead of only guarding the null-URL case. All five call sites pass an
explicit size modifier, so the new Box wrapper is layout-safe.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/components/CompactTrackCard.svelte

@@ -2,9 +2,10 @@
   import { Plus } from 'lucide-svelte';
   import type { TrackRef } from '$lib/api/types';
   import { playQueue, enqueueTrack } from '$lib/player/store.svelte';
-  import { FALLBACK_COVER, coverUrl } from '$lib/media/covers';
+  import { coverUrl } from '$lib/media/covers';
   import TrackMenu from './TrackMenu.svelte';
   import LikeButton from './LikeButton.svelte';
+  import Cover from './Cover.svelte';
 
   // Horizontal compact track row — cover thumb on the left, title +
   // artist on the right. Mirrors Android's CompactTrackTile so the
@@ -23,10 +24,6 @@
 
   const cover = $derived(coverUrl(track.album_id));
 
-  function onImgError(e: Event) {
-    (e.currentTarget as HTMLImageElement).src = FALLBACK_COVER;
-  }
-
   function onClick() {
     playQueue(sectionTracks, index);
   }
@@ -47,13 +44,7 @@
            hover:bg-surface-hover focus-visible:ring-2 focus-visible:ring-accent"
   >
     <div class="h-12 w-12 flex-shrink-0 overflow-hidden rounded bg-surface-hover">
-      <img
-        src={cover}
-        alt=""
-        class="h-full w-full object-cover"
-        loading="lazy"
-        onerror={onImgError}
-      />
+      <Cover src={cover} />
     </div>
     <div class="min-w-0 flex-1">
       <div class="truncate text-sm font-medium text-text-primary">{track.title}</div>