Fix Android Auto seek buttons doing nothing for car-initiated playback (#466)

Two issues prevented the head unit seek controls from working:

1. Books started from the Android Auto browse screen never populated
   MediaRepository's playingBook state, which is only set by the phone
   UI flow. Since rewind/forward/previousTrack/nextTrack all bail out
   silently on a null playingBook, every seek button on the head unit
   was a no-op while pause kept working (it bypasses MediaRepository).
   onSetMediaItems now registers the fetched book with MediaRepository.

2. The rewind/forward buttons were assigned to SLOT_OVERFLOW, leaving
   the primary back/forward slots to chapter navigation. Swapped so the
   configurable rewind/forward occupy the primary slots, matching
   audiobook app conventions, with chapter navigation in the overflow.

Verified end-to-end on the Desktop Head Unit: commands confirmed
arriving via logcat and seeks now applied for books started from the
car screen.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/src/main/kotlin/org/grakovne/lissen/playback/MediaLibrarySessionCallback.kt

@@ -113,7 +113,7 @@ class MediaLibrarySessionCallback
           .setSessionCommand(prevChapterCommand)
           .setDisplayName("Previous Chapter")
           .setEnabled(true)
-          .setSlots(CommandButton.SLOT_BACK)
+          .setSlots(CommandButton.SLOT_OVERFLOW)
           .build()
 
       val nextChapterButton =
@@ -121,7 +121,7 @@ class MediaLibrarySessionCallback
           .Builder(CommandButton.ICON_NEXT)
           .setSessionCommand(nextChapterCommand)
           .setDisplayName("Next Chapter")
-          .setSlots(CommandButton.SLOT_FORWARD)
+          .setSlots(CommandButton.SLOT_OVERFLOW)
           .setEnabled(true)
           .build()
 
@@ -131,7 +131,7 @@ class MediaLibrarySessionCallback
           .setSessionCommand(rewindCommand)
           .setDisplayName("Rewind")
           .setEnabled(true)
-          .setSlots(CommandButton.SLOT_OVERFLOW)
+          .setSlots(CommandButton.SLOT_BACK)
           .build()
 
       val forwardButton =
@@ -139,7 +139,7 @@ class MediaLibrarySessionCallback
           .Builder(CommandButton.ICON_SKIP_FORWARD)
           .setSessionCommand(forwardCommand)
           .setDisplayName("Forward")
-          .setSlots(CommandButton.SLOT_OVERFLOW)
+          .setSlots(CommandButton.SLOT_FORWARD)
           .setEnabled(true)
           .build()
 
@@ -210,6 +210,7 @@ class MediaLibrarySessionCallback
                       preferences.savePlayingItem(it)
                       playbackSynchronizationService.startPlaybackSynchronization(it)
                     }
+                    mediaRepository.registerPlayingBook(it)
                     PlaybackService.bookToChapterMediaItems(it)
                   },
                   onFailure = { MediaItemsWithStartPosition(emptyList(), 0, 0) },

app/src/main/kotlin/org/grakovne/lissen/playback/MediaRepository.kt

@@ -415,6 +415,18 @@ class MediaRepository
       _isPlaybackReady.value = false
     }
 
+    fun registerPlayingBook(book: DetailedItem) {
+      val sameBook = _playingBook.value?.same(book) ?: false
+
+      if (sameBook.not()) {
+        Timber.d("Registering playing book prepared via media session: ${book.id}")
+
+        _totalPosition.value = book.progress?.currentTime ?: 0.0
+        _playingBook.value = book
+        _isPlaybackReady.value = true
+      }
+    }
+
     private fun startPreparingPlayback(book: DetailedItem) {
       val sameBook = _playingBook.value?.same(book) ?: false