split the song projections; the queue carries the playlist key

`SongListItem` carried a nullable `position` that only playlist rows have,
so the album/song queries projected a fake `NULL AS position` just to map
the shared type — a leak of the playlist concept into unrelated reads.

Split it into `AlbumSongItem` (song, coverArt) and `PlaylistSongItem` (song,
coverArt, position), plus a `SongItem` interface so the queue stays generic
over kinds. The queue's `QueueWindowItem` now carries the optional
`playlistPosition` for the keyset cursor, so no projection fabricates it.
Playlist detail and its paging use `PlaylistSongItem`.

app/src/test/kotlin/com/subtracks/data/repo/QueueKeysetTest.kt

@@ -146,7 +146,7 @@ class QueueKeysetTest {
             val actual =
                 db
                     .queueDao()
-                    .playlistSongsFrom(1, "pl1", anchor.position!!, skip = 0, limit = 5)
+                    .playlistSongsFrom(1, "pl1", anchor.position, skip = 0, limit = 5)
                     .map { it.song.id }
             assertEquals(expected, actual)
         }
@@ -234,7 +234,7 @@ class QueueKeysetTest {
             seedLibrary()
             val plan =
                 queryPlan(
-                    "SELECT songs.*, albums.coverArt AS coverArt, NULL AS position FROM songs " +
+                    "SELECT songs.*, albums.coverArt AS coverArt FROM songs " +
                         "LEFT JOIN albums ON albums.sourceId = songs.sourceId AND albums.id = songs.albumId " +
                         "WHERE songs.sourceId = 1 AND songs.albumId = 'big' " +
                         "AND (songs.disc, songs.track, songs.id) >= (1, 1, 'b0') " +

app/src/test/kotlin/com/subtracks/ui/AppScreenshotTest.kt

@@ -33,8 +33,8 @@ import com.subtracks.data.model.Album
 import com.subtracks.data.model.Artist
 import com.subtracks.data.model.CoverArtRef
 import com.subtracks.data.model.Playlist
+import com.subtracks.data.model.PlaylistSongItem
 import com.subtracks.data.model.Song
-import com.subtracks.data.model.SongListItem
 import com.subtracks.data.model.Source
 import com.subtracks.data.prefs.StreamQuality
 import com.subtracks.playback.PlaybackState
@@ -530,8 +530,12 @@ private object Fixtures {
         )
 
     val songItems =
-        songs.map { song ->
-            SongListItem(song = song, coverArt = albums.firstOrNull { it.id == song.albumId }?.coverArt)
+        songs.mapIndexed { index, song ->
+            PlaylistSongItem(
+                song = song,
+                coverArt = albums.firstOrNull { it.id == song.albumId }?.coverArt,
+                position = index.toLong(),
+            )
         }
 
     val playlists =

app/src/test/kotlin/com/subtracks/ui/playback/QueueViewModelTest.kt

@@ -8,9 +8,9 @@ import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.subtracks.data.db.SubtracksDatabase
 import com.subtracks.data.download.ArtworkStore
 import com.subtracks.data.download.FakeDownloadEngine
+import com.subtracks.data.model.AlbumSongItem
 import com.subtracks.data.model.QueueKind
 import com.subtracks.data.model.Song
-import com.subtracks.data.model.SongListItem
 import com.subtracks.data.model.Source
 import com.subtracks.data.prefs.fakeUserPreferences
 import com.subtracks.data.repo.DownloadRepository
@@ -408,7 +408,7 @@ class QueueViewModelTest {
         QueueRow(
             position.toLong(),
             position.toLong(),
-            SongListItem(
+            AlbumSongItem(
                 song =
                     Song(
                         sourceId = 1,