feat(android): ArtistDetail avatar falls back to first album cover (parity map)

Ports flutter_client/lib/library/artist_detail_screen.dart:177-208
_ArtistAvatar: server coverUrl wins; when empty, use the first loaded
album's /api/albums/{id}/cover; only show the bare Lucide.User icon
when the artist has neither. Threads detail.albums.firstOrNull()?.id
through ArtistHeader → ArtistAvatar. (Seeded-loading header passes
null since the seed carries no album list.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/library/ui/ArtistDetailScreen.kt

@@ -141,6 +141,7 @@ private fun ArtistBody(
             ArtistHeader(
                 artist = detail.artist,
                 artistLiked = artistLiked,
+                fallbackAlbumId = detail.albums.firstOrNull()?.id,
                 onPlay = onPlay,
                 onToggleLike = onToggleLike,
             )
@@ -161,6 +162,7 @@ private fun ArtistBody(
 private fun ArtistHeader(
     artist: ArtistRef,
     artistLiked: Boolean,
+    fallbackAlbumId: String?,
     onPlay: () -> Unit,
     onToggleLike: () -> Unit,
 ) {
@@ -171,7 +173,7 @@ private fun ArtistHeader(
         verticalAlignment = Alignment.CenterVertically,
         horizontalArrangement = Arrangement.spacedBy(12.dp),
     ) {
-        ArtistAvatar(artist)
+        ArtistAvatar(artist, fallbackAlbumId)
         Column(modifier = Modifier.weight(1f)) {
             Text(
                 text = artist.name,
@@ -198,7 +200,16 @@ private fun ArtistHeader(
 }
 
 @Composable
-private fun ArtistAvatar(artist: ArtistRef) {
+private fun ArtistAvatar(artist: ArtistRef, fallbackAlbumId: String? = null) {
+    // Server coverUrl wins; otherwise mirror the server's "use the
+    // first album's cover" rule via the loaded album list (Flutter
+    // _ArtistAvatar). Bare Lucide.User only when neither exists.
+    val model = when {
+        artist.coverUrl.isNotEmpty() -> artist.coverUrl
+        fallbackAlbumId != null ->
+            "http://placeholder.invalid/api/albums/$fallbackAlbumId/cover"
+        else -> null
+    }
     Box(
         modifier = Modifier
             .size(96.dp)
@@ -206,7 +217,7 @@ private fun ArtistAvatar(artist: ArtistRef) {
             .background(MaterialTheme.colorScheme.surfaceVariant),
         contentAlignment = Alignment.Center,
     ) {
-        if (artist.coverUrl.isEmpty()) {
+        if (model == null) {
             Icon(
                 imageVector = Lucide.User,
                 contentDescription = null,
@@ -215,7 +226,7 @@ private fun ArtistAvatar(artist: ArtistRef) {
             )
         } else {
             AsyncImage(
-                model = artist.coverUrl,
+                model = model,
                 contentDescription = artist.name,
                 modifier = Modifier.fillMaxSize(),
                 contentScale = ContentScale.Crop,