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/AlbumCard.svelte

@@ -1,18 +1,14 @@
 <script lang="ts">
   import type { AlbumRef, AlbumDetail } from '$lib/api/types';
-  import { FALLBACK_COVER } from '$lib/media/covers';
   import { api } from '$lib/api/client';
   import { enqueueTracks, playQueue } from '$lib/player/store.svelte';
   import { Play } from 'lucide-svelte';
   import AlbumMenu from './AlbumMenu.svelte';
   import CardActionCluster from './CardActionCluster.svelte';
+  import Cover from './Cover.svelte';
 
   let { album }: { album: AlbumRef } = $props();
 
-  function onImgError(e: Event) {
-    (e.currentTarget as HTMLImageElement).src = FALLBACK_COVER;
-  }
-
   async function onAddClick(e: MouseEvent) {
     e.preventDefault();
     e.stopPropagation();
@@ -38,12 +34,9 @@
              shadow-sm transition-all duration-150
              group-hover:shadow-lg group-hover:ring-1 group-hover:ring-accent/40"
     >
-      <img
+      <Cover
         src={album.cover_url}
-        alt=""
-        class="h-full w-full object-cover transition-transform group-hover:scale-[1.03]"
-        loading="lazy"
-        onerror={onImgError}
+        class="transition-transform group-hover:scale-[1.03]"
       />
       <button
         type="button"