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/ArtistCard.svelte

@@ -2,9 +2,10 @@
   import type { ArtistRef, TrackRef } from '$lib/api/types';
   import { api } from '$lib/api/client';
   import { playQueue, enqueueTracks } from '$lib/player/store.svelte';
-  import { Disc3, Play } from 'lucide-svelte';
+  import { Play } from 'lucide-svelte';
   import ArtistMenu from './ArtistMenu.svelte';
   import CardActionCluster from './CardActionCluster.svelte';
+  import Cover from './Cover.svelte';
   import { listArtistTracks } from '$lib/api/artists';
 
   let { artist }: { artist: ArtistRef } = $props();
@@ -48,18 +49,12 @@
     class="block rounded focus-visible:ring-2 focus-visible:ring-accent"
   >
     <div class="art-wrap relative mx-auto aspect-square w-full overflow-hidden rounded-full bg-surface-hover">
-      {#if artist.cover_url}
-        <img
-          src={artist.cover_url}
-          alt=""
-          class="h-full w-full object-cover transition-transform group-hover:scale-[1.03]"
-          loading="lazy"
-        />
-      {:else}
-        <div class="flex h-full w-full items-center justify-center">
-          <Disc3 strokeWidth={1} class="h-4/5 w-4/5 text-text-muted" />
-        </div>
-      {/if}
+      <Cover
+        src={artist.cover_url}
+        shape="round"
+        fallback="artist"
+        class="transition-transform group-hover:scale-[1.03]"
+      />
       <button
         type="button"
         aria-label={`Play ${artist.name}`}