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)
         {