feat: enhance audio playback features and UI, including audio player dialog and improved file size handling

src/EchoHub.Core/Constants/HubConstants.cs

@@ -7,6 +7,7 @@ public static class HubConstants
     public const int DefaultHistoryCount = 100;
     public const int MaxMessageLength = 2000;
     public const int MaxFileSizeBytes = 10 * 1024 * 1024; // 10 MB
+    public const int MaxAudioFileSizeBytes = 20 * 1024 * 1024; // 20 MB
     public const int MaxAvatarSizeBytes = 2 * 1024 * 1024; // 2 MB
     public const int MaxMessageNewlines = 30;
     public const int MaxConsecutiveNewlines = 1;