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/Clients/TrackMediaClient.cs

@@ -11,12 +11,20 @@ public class TrackMediaResponse : IDisposable
 {
     public Stream Stream { get; }
     public long ContentLength { get; }
+
+    /// <summary>
+    /// The response media type (e.g. "audio/wav", "audio/mpeg"). Drives format-decoder
+    /// selection on the JS side. Falls back to "audio/wav" when the server omits the header.
+    /// </summary>
+    public string ContentType { get; }
+
     private readonly HttpResponseMessage _response;
 
-    public TrackMediaResponse(Stream stream, long contentLength, HttpResponseMessage response)
+    public TrackMediaResponse(Stream stream, long contentLength, string contentType, HttpResponseMessage response)
     {
         Stream = stream;
         ContentLength = contentLength;
+        ContentType = contentType;
         _response = response;
     }
 
@@ -61,11 +69,14 @@ public class TrackMediaClient
             response.EnsureSuccessStatusCode();
 
             var contentLength = response.Content.Headers.ContentLength ?? 0;
+            // Default to WAV when the server omits the header — the only format shipping
+            // today — so the JS factory always receives a usable media type.
+            var contentType = response.Content.Headers.ContentType?.MediaType ?? "audio/wav";
             var stream = await response.Content.ReadAsStreamAsync(cancellationToken);
 
             // TrackMediaResponse takes ownership of both stream and response;
             // do NOT dispose response here — the caller disposes via TrackMediaResponse.Dispose().
-            return ApiResult<TrackMediaResponse>.CreatePassResult(new TrackMediaResponse(stream, contentLength, response));
+            return ApiResult<TrackMediaResponse>.CreatePassResult(new TrackMediaResponse(stream, contentLength, contentType, response));
         }
         catch (Exception e)
         {

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}";