feat: Deezer artist photo fallback for local artists with no Navidrome image

Niche local artists (e.g. N!GHT) whose Navidrome metadata agents have no image showed a
blank hero. Now, when Navidrome has no artist image, the hero falls back to the matched
Deezer artist's photo (Navidrome image → Deezer photo → album cover), on web + Android.

- Web (detail.js): the Deezer-photo fallback previously ran only for non-local artists;
  now it runs whenever the Navidrome image is absent, local or not.
- Android: ArtistViewModel captures the matched Deezer artist's photo (deezerImage);
  ArtistDetailScreen uses it as the hero fallback.
SW ns-v56.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/screen/ArtistDetailScreen.kt

@@ -218,7 +218,10 @@ fun ArtistDetailScreen(
 
         // Hero card
         item {
+            // Navidrome's own artist image first; if its agents have none (niche artists like N!GHT),
+            // fall back to the matched Deezer artist photo, then the album cover.
             val imgUrl = state.info?.largeImageUrl?.takeIf { it.isNotBlank() }
+                ?: state.deezerImage
                 ?: coverUrl(artist.coverArt)
             Box(
                 Modifier

android/app/src/main/kotlin/com/novasonic/app/viewmodel/ArtistViewModel.kt

@@ -43,6 +43,8 @@ data class ArtistUiState(
     val showEventsSheet: Boolean = false,
     // Deezer fan count of the matched artist (shown under the name), null if unknown
     val fanCount: Long? = null,
+    // Deezer artist photo — hero fallback when Navidrome's agents have no image (niche artists)
+    val deezerImage: String? = null,
     // Preview + download
     val previewingId: Long? = null,
     val downloadSheetItem: ItunesItem? = null,
@@ -121,7 +123,10 @@ class ArtistViewModel(app: Application, savedState: SavedStateHandle) : AndroidV
                 ?: artists.firstOrNull()
         )
         val iArtistId = iArtist?.artistId
-        if (iArtist?.fanCount != null) _state.value = _state.value.copy(fanCount = iArtist.fanCount)
+        if (iArtist != null) _state.value = _state.value.copy(
+            fanCount    = iArtist.fanCount ?: _state.value.fanCount,
+            deezerImage = iArtist.artworkUrl100?.takeIf { it.isNotBlank() } ?: _state.value.deezerImage,
+        )
 
         if (iArtistId != null) {
             coroutineScope {