feat(android): surface queue source on PlayerUiState

Reads MINSTREL_SOURCE_KEY from the current MediaItem's extras and
projects it as PlayerUiState.currentSource. PlayEventsReporter
needs this to tag plays with their originating system playlist
('for_you'/'discover'/'radio:<id>') so the server advances the
right rotation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -156,6 +156,10 @@ class PlayerController @Inject constructor(
                 override fun onEvents(player: Player, events: Player.Events) {
                     val idx = player.currentMediaItemIndex
                     val current = queueRefs.getOrNull(idx)
+                    val source = player.currentMediaItem
+                        ?.mediaMetadata
+                        ?.extras
+                        ?.getString(MINSTREL_SOURCE_KEY)
                     uiStateInternal.value =
                         PlayerUiState(
                             currentTrack = current,
@@ -167,6 +171,7 @@ class PlayerController @Inject constructor(
                             durationMs = player.duration.coerceAtLeast(0),
                             bufferedPositionMs = player.bufferedPosition.coerceAtLeast(0),
                             playbackError = player.playerError?.message,
+                            currentSource = source,
                         )
                 }
             },

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

@@ -18,4 +18,5 @@ data class PlayerUiState(
     val durationMs: Long = 0,
     val bufferedPositionMs: Long = 0,
     val playbackError: String? = null,
+    val currentSource: String? = null,
 )