fix(android): AddToPlaylist routes empty list through UiState.Empty

The original AddToPlaylistUiState only had Loading/Success/Error, with
the empty-list message branched inside Success. After migrating to
shared UiState<T> (which adds Empty), the sheet's when over the state
was non-exhaustive. Route empty through UiState.Empty at the VM, drop
the inner branch in the sheet, and add the explicit Empty arm in the
when block to satisfy the compiler.

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

@@ -60,22 +60,19 @@ fun AddToPlaylistSheet(
                     modifier = Modifier.fillMaxWidth().padding(24.dp),
                     contentAlignment = Alignment.Center,
                 ) { CircularProgressIndicator(color = MaterialTheme.colorScheme.primary) }
+                UiState.Empty -> Text(
+                    text = "You haven't created any playlists yet.",
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                    modifier = Modifier.padding(horizontal = 20.dp, vertical = 12.dp),
+                )
                 is UiState.Error -> Text(
                     text = s.message,
                     color = MaterialTheme.colorScheme.error,
                     modifier = Modifier.padding(horizontal = 20.dp, vertical = 12.dp),
                 )
-                is UiState.Success -> if (s.data.isEmpty()) {
-                    Text(
-                        text = "You haven't created any playlists yet.",
-                        color = MaterialTheme.colorScheme.onSurfaceVariant,
-                        modifier = Modifier.padding(horizontal = 20.dp, vertical = 12.dp),
-                    )
-                } else {
-                    LazyColumn(modifier = Modifier.fillMaxWidth()) {
-                        items(items = s.data, key = { it.id }) { p ->
-                            PlaylistPickRow(playlist = p, onClick = { onPick(p.id) })
-                        }
+                is UiState.Success -> LazyColumn(modifier = Modifier.fillMaxWidth()) {
+                    items(items = s.data, key = { it.id }) { p ->
+                        PlaylistPickRow(playlist = p, onClick = { onPick(p.id) })
                     }
                 }
             }

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

@@ -30,7 +30,9 @@ class AddToPlaylistViewModel @Inject constructor(
 
     val uiState: StateFlow<UiState<List<PlaylistRef>>> = repository
         .observeUserPlaylists()
-        .map { list -> UiState.Success(list) as UiState<List<PlaylistRef>> }
+        .map { list ->
+            if (list.isEmpty()) UiState.Empty else UiState.Success(list)
+        }
         .onStart {
             // Best-effort refresh so the sheet reflects newly-created
             // playlists from another device. Cached list shows first;