refactor(audio): extract IFormatDecoder/WavFormatDecoder and wire Content-Type to JS format selection

StreamDecoder is now format-agnostic; WavFormatDecoder delegates to WavUtils; contentType flows C# to JS.

DeepDrftPublic.Client/Services/AudioInteropService.cs

@@ -65,9 +65,9 @@ public class AudioInteropService : IAsyncDisposable
     }
 
     // Streaming methods
-    public async Task<AudioOperationResult> InitializeStreaming(string playerId, long totalStreamLength)
+    public async Task<AudioOperationResult> InitializeStreaming(string playerId, long totalStreamLength, string contentType)
     {
-        return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.initializeStreaming", playerId, totalStreamLength);
+        return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.initializeStreaming", playerId, totalStreamLength, contentType);
     }
 
     public async Task<StreamingResult> ProcessStreamingChunk(string playerId, byte[] audioChunk)

DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs

@@ -143,8 +143,9 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
 
             using var audio = mediaResult.Value;
 
-            // Initialize streaming mode with content length
-            var streamingResult = await _audioInterop.InitializeStreaming(PlayerId, audio.ContentLength);
+            // Initialize streaming mode with content length and media type (drives
+            // JS format-decoder selection).
+            var streamingResult = await _audioInterop.InitializeStreaming(PlayerId, audio.ContentLength, audio.ContentType);
             if (!streamingResult.Success)
             {
                 var technicalError = $"Failed to initialize streaming: {streamingResult.Error}";