fix(player): route notification next/prev to Sonos while casting — #171

Device verification of v2026.07.15 found the notification/lock-screen
next+prev buttons dead during a UPnP cast (play/pause worked). Root cause:
the system media controls issue COMMAND_SEEK_TO_NEXT / COMMAND_SEEK_TO_PREVIOUS
-> Player.seekToNext()/seekToPrevious(), which are DISTINCT from the
seekToNextMediaItem()/seekToPreviousMediaItem() the in-app buttons call and
which MinstrelForwardingPlayer already routes to Sonos. seekToNext/Previous
were un-overridden, so ForwardingPlayer forwarded them to the paused local
delegate — nudging its cursor, which the identity poll then re-synced back to
Sonos, so the buttons read as dead.

Override seekToNext()/seekToPrevious() to delegate to the media-item variants
(the full Sonos path: optimistic local advance + AVTransport Next/Previous +
pending-transport gate) when a UPnP route is engaged; plain local playback
keeps the default behaviour. Fixes notification/lock-screen/Auto/Wear skip
during a cast. Completes milestone #171 Step 3 (#1606 / #606).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/player/MinstrelForwardingPlayer.kt

@@ -342,6 +342,23 @@ class MinstrelForwardingPlayer(
         }
     }
 
+    // The system media notification / lock-screen / Android Auto / Wear 'next'
+    // and 'previous' buttons issue COMMAND_SEEK_TO_NEXT / COMMAND_SEEK_TO_PREVIOUS
+    // -> Player.seekToNext() / seekToPrevious(), which are DISTINCT from the
+    // *MediaItem variants the in-app transport buttons call. Un-overridden, the
+    // base ForwardingPlayer forwards these to the paused local delegate -- so
+    // the notification next/prev only nudged the local cursor (which the poll
+    // then re-synced back to Sonos), reading as dead buttons while casting.
+    // Route them through the same Sonos path as the media-item variants when a
+    // UPnP route is engaged; plain local playback keeps the default behaviour.
+    override fun seekToNext() {
+        if (isRemote() || isLoadingUpnp()) seekToNextMediaItem() else super.seekToNext()
+    }
+
+    override fun seekToPrevious() {
+        if (isRemote() || isLoadingUpnp()) seekToPreviousMediaItem() else super.seekToPrevious()
+    }
+
     override fun getCurrentPosition(): Long =
         if (isRemote()) remoteState.positionMs else super.getCurrentPosition()