songs type label, disc header alignment, now playing back test

- Now Playing: the type line reads "songs" for the library songs list, source line stays "Library"
- Album detail: align the disc icon over the track numbers and the disc label with the track titles (16dp start + 18dp icon + 16dp gap, matching the ListItem columns)
- Add NowPlayingBackTest covering back-from-overlay not popping the destination

app/src/main/kotlin/com/subtracks/ui/library/AlbumDetailScreen.kt

@@ -149,15 +149,15 @@ fun AlbumDetailScreen(
 private fun DiscHeader(text: String) {
     Row(
         verticalAlignment = Alignment.CenterVertically,
-        modifier = Modifier.padding(start = 8.dp, end = 16.dp, top = 28.dp, bottom = 4.dp),
+        modifier = Modifier.padding(start = 16.dp, end = 16.dp, top = 28.dp, bottom = 4.dp),
     ) {
         Icon(
             imageVector = Icons.Outlined.FiberSmartRecord,
             contentDescription = null,
             tint = MaterialTheme.colorScheme.onSurfaceVariant,
-            modifier = Modifier.size(20.dp),
+            modifier = Modifier.size(18.dp),
         )
-        Spacer(Modifier.width(8.dp))
+        Spacer(Modifier.width(16.dp))
         Text(
             text = text.uppercase(),
             style = MaterialTheme.typography.labelLarge,

app/src/main/kotlin/com/subtracks/ui/playback/NowPlayingScreen.kt

@@ -402,7 +402,7 @@ private fun QueueKind.label(): String =
         QueueKind.Album -> "album"
         QueueKind.Playlist -> "playlist"
         QueueKind.Song -> "song"
-        QueueKind.Songs -> "library"
+        QueueKind.Songs -> "songs"
     }
 
 private fun formatTime(milliseconds: Long): String {