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>

android/app/src/main/java/com/fabledsword/minstrel/shared/widgets/ServerImage.kt

@@ -1,15 +1,25 @@
 package com.fabledsword.minstrel.shared.widgets
 
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.layout.ContentScale
 import coil3.compose.AsyncImage
+import coil3.compose.AsyncImagePainter
 import com.fabledsword.minstrel.shared.resolveServerUrl
 
 /**
  * Renders a server-hosted image, resolving relative URLs centrally so
  * every cover surface loads consistently. Shows [fallback] when the URL
- * is blank or unresolvable.
+ * is blank/unresolvable, while the image is still loading, and when the
+ * load fails — so a tile is never left blank (e.g. art not yet backfilled,
+ * which the "You might like" row hits often).
  */
 @Composable
 fun ServerImage(
@@ -22,12 +32,26 @@ fun ServerImage(
     val resolved = resolveServerUrl(url)
     if (resolved == null) {
         fallback()
-    } else {
+        return
+    }
+    // Track Coil's load state so the fallback doubles as a placeholder
+    // (loading) and an error state (404 / unreachable) — not just a
+    // null-URL guard, which left present-but-failing URLs blank.
+    var state by remember(resolved) {
+        mutableStateOf<AsyncImagePainter.State>(AsyncImagePainter.State.Empty)
+    }
+    Box(modifier = modifier, contentAlignment = Alignment.Center) {
         AsyncImage(
             model = resolved,
             contentDescription = contentDescription,
-            modifier = modifier,
+            modifier = Modifier.fillMaxSize(),
             contentScale = contentScale,
+            onState = { state = it },
         )
+        if (state is AsyncImagePainter.State.Loading ||
+            state is AsyncImagePainter.State.Error
+        ) {
+            fallback()
+        }
     }
 }