Reset player aspect ratio to 16:9 for audio-only playback

Audio-only streams never report a video size, so the aspect ratio of
the last played video stuck to the player area indefinitely once audio
mode was enabled. Reset it in play() when the selected stream is
audio-only, and let the macOS player window fall back to 16:9 instead
of keeping the previous lock.

Yattee/Services/Player/PlayerService.swift

@@ -347,6 +347,13 @@ final class PlayerService {
                 }
             }
 
+            // Audio-only streams never report a video size, so the previous
+            // video's aspect ratio would stick to the player area forever.
+            // Reset it so the UI falls back to the standard 16:9.
+            if selectedStream.isAudioOnly {
+                state.videoAspectRatio = nil
+            }
+
             // Check for cancellation before loading stream
             try Task.checkCancellation()