refactor(PlaybackState): add URL context to Connecting state

- Add urls: List<String> = emptyList() for URL fallback list
- Add currentUrlIndex: Int = 0 for tracking fallback progress
- Defaults preserve backward compatibility (Connecting(station) still works)

Made-with: Cursor

app/src/main/java/xyz/cottongin/radio247/service/PlaybackState.kt

@@ -8,6 +8,8 @@ sealed interface PlaybackState {
     data object Idle : PlaybackState
     data class Connecting(
         val station: Station,
+        val urls: List<String> = emptyList(),
+        val currentUrlIndex: Int = 0,
         val sessionStartedAt: Long = System.currentTimeMillis()
     ) : PlaybackState
     data class Playing(