fix shuffled queue edits: flat remap, pinned shuffle start, stable highlight

The previous commit assumed a removed/moved track only shifts play
positions by one, but a shuffled context removal re-derives the
permutation, so the current track's play position is a remap of its flat
index. `removeAt` now maps the current's flat through the updated order
(shuffled) or shifts it (unshuffled), and re-anchors the up-next block to
the track the cursor lands on. This fixes removing any track while
shuffled picking a random track and the anchor removal not advancing to
the block.

Restore "shuffle starts at the top": pick the seed so the requested start
track maps to play position 0 (`pinnedSeed`), instead of leaving the
current at its derived position. Applies to shuffle-play, tapping a track
while shuffled, toggling shuffle on, and restore.

The queue view's playing highlight now requires both the combined position
and the song id, so a transient position mismatch no longer flashes the
row above as playing.

app/src/test/kotlin/com/subtracks/playback/PlaybackControllerTest.kt

@@ -2003,6 +2003,76 @@ class PlaybackControllerTest {
         )
     }
 
+    @Test
+    fun shufflePlayStartsAtTheTopOfTheQueue() {
+        seedAlbum(20, sourceId = 1)
+
+        controller.shuffleAlbum(1, "al1")
+        await { controller.state.value.item != null && controller.state.value.shuffle }
+
+        assertEquals(0L, controller.state.value.position)
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(
+            controller.state.value.item
+                ?.id,
+            runBlocking { queues.itemAt(snapshot, 0)?.song?.id },
+        )
+    }
+
+    @Test
+    fun enablingShufflePinsThePlayingTrackToTheTop() {
+        seedAlbum(20, sourceId = 1)
+
+        controller.playAlbum(1, "al1", 7)
+        await {
+            controller.state.value.item
+                ?.id == "s8"
+        }
+
+        controller.toggleShuffle()
+        await { controller.state.value.shuffle }
+
+        assertEquals(0L, controller.state.value.position)
+        assertEquals(
+            "s8",
+            controller.state.value.item
+                ?.id,
+        )
+    }
+
+    @Test
+    fun removingAContextTrackWhileShuffledKeepsThePlayingTrackAndTheBlock() {
+        seedAlbum(6, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 2)
+        await {
+            controller.state.value.item
+                ?.id == "s3"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+        controller.toggleShuffle()
+        await { controller.state.value.shuffle && runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        val currentId =
+            controller.state.value.item!!
+                .id
+        val before = runBlocking { queues.snapshot() }
+        val currentPosition = controller.state.value.position!!
+        val removePosition = (0 until before.size).first { !before.isUpNext(it) && it != currentPosition }
+
+        runBlocking { controller.removeAt(removePosition) }
+
+        await {
+            val snap = runBlocking { queues.snapshot() }
+            controller.state.value.item
+                ?.id == currentId &&
+                runBlocking { queues.itemAt(snap, controller.state.value.position!!)?.song?.id } == currentId
+        }
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+    }
+
     @Test
     fun shufflingTheSameAlbumAgainPicksADifferentTrack() {
         seedAlbum(5, sourceId = 1)

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

@@ -416,6 +416,7 @@ class AppScreenshotTest {
                         Fixtures.songItems.mapIndexed { index, item -> QueueRow(index.toLong(), index.toLong(), item) },
                     ready = true,
                     currentPosition = 1L,
+                    currentSongId = Fixtures.songItems[1].song.id,
                     coverArt = { id, _ -> id?.let { CoverArtRef(it, "test:$it") } },
                     onBack = {},
                     onPlay = {},