fix(android): ArtistDetail Play button surfaces failures (audit v3 Bug-7)

playArtist() previously swallowed all exceptions with a comment
about a 'future refinement' — if the tracks fetch failed or the
artist had no playable tracks, the button looked broken with no
feedback.

Now: ArtistDetailViewModel exposes a transientMessages Flow backed
by a buffered Channel. playArtist sends "Couldn't start playback:
<reason>" on Throwable and "No tracks to play for this artist."
when the result is empty. ArtistDetailScreen collects the Flow into
a Scaffold-level SnackbarHost so users see the failure inline
instead of tapping a dead button repeatedly.

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

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

@@ -21,6 +21,8 @@ import androidx.compose.foundation.lazy.grid.items
 import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.material3.Button
 import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.SnackbarHostState
 import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
@@ -28,7 +30,9 @@ import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
@@ -59,6 +63,12 @@ fun ArtistDetailScreen(
     viewModel: ArtistDetailViewModel = hiltViewModel(),
 ) {
     val state by viewModel.uiState.collectAsStateWithLifecycle()
+    val snackbarHostState = remember { SnackbarHostState() }
+    LaunchedEffect(Unit) {
+        viewModel.transientMessages.collect { msg ->
+            snackbarHostState.showSnackbar(msg)
+        }
+    }
     Scaffold(
         modifier = Modifier.fillMaxSize(),
         topBar = {
@@ -71,6 +81,7 @@ fun ArtistDetailScreen(
                 },
             )
         },
+        snackbarHost = { SnackbarHost(snackbarHostState) },
     ) { inner ->
         PullToRefreshScaffold(
             onRefresh = { viewModel.refresh().join() },

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

@@ -13,10 +13,13 @@ import com.fabledsword.minstrel.nav.DetailSeedCache
 import com.fabledsword.minstrel.player.PlayerController
 import dagger.hilt.android.lifecycle.HiltViewModel
 import kotlinx.coroutines.Job
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.receiveAsFlow
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
 import javax.inject.Inject
@@ -45,6 +48,10 @@ class ArtistDetailViewModel @Inject constructor(
         MutableStateFlow<ArtistDetailUiState>(ArtistDetailUiState.Loading())
     val uiState: StateFlow<ArtistDetailUiState> = internal.asStateFlow()
 
+    /** Transient one-shot messages for the screen-level snackbar (e.g. Play-button failure). */
+    private val transientMessagesChannel = Channel<String>(Channel.BUFFERED)
+    val transientMessages: Flow<String> = transientMessagesChannel.receiveAsFlow()
+
     val artistLiked: StateFlow<Boolean> =
         likes.observeIsLiked(LikesRepository.ENTITY_ARTIST, artistId)
             .stateIn(
@@ -92,7 +99,9 @@ class ArtistDetailViewModel @Inject constructor(
         viewModelScope.launch {
             try {
                 val tracks = repository.fetchArtistTracks(artistId).shuffled()
-                if (tracks.isNotEmpty()) {
+                if (tracks.isEmpty()) {
+                    transientMessagesChannel.trySend("No tracks to play for this artist.")
+                } else {
                     player.setQueue(
                         tracks = tracks,
                         initialIndex = 0,
@@ -100,11 +109,11 @@ class ArtistDetailViewModel @Inject constructor(
                     )
                 }
             } catch (
-                @Suppress("TooGenericExceptionCaught", "SwallowedException") e: Throwable,
+                @Suppress("TooGenericExceptionCaught") e: Throwable,
             ) {
-                // Silent failure — the play button has no inline error
-                // affordance; user can retry by re-tapping. A future
-                // refinement surfaces this via a snackbar.
+                transientMessagesChannel.trySend(
+                    "Couldn't start playback: ${e.message ?: "unknown error"}",
+                )
             }
         }
     }