feat: replace ?offset= seek with HTTP Range streaming across API, proxy, and client

- API: enableRangeProcessing true on no-offset FileStream path
- Proxy: transparent Range relay, forwards 206/416/Content-Range verbatim
- TrackMediaClient: Range: bytes=X- replaces ?offset=X; response disposed via TrackMediaResponse
- StreamDecoder: reinitializeForRangeContinuation retains wavHeader, counts raw PCM against 206 Content-Length
- AudioPlayer: seekBeyondBuffer adds headerSize for file-absolute offset; duration guard prevents continuation overwriting full-track duration
- StreamingAudioPlayerService: seek guard corrected to >= 0 (file-absolute offset contract)

DeepDrftAPI/Controllers/TrackController.cs

@@ -403,8 +403,10 @@ public class TrackController : ControllerBase
                 _logger.LogInformation(
                     "Streaming track from disk: {TrackId}, Size: {Size} bytes",
                     trackId, streamLength);
-                // enableRangeProcessing: false — seek is served by WavOffsetService, not Range.
-                return File(innerStream, streamMimeType, enableRangeProcessing: false);
+                // enableRangeProcessing: true — seek is served by HTTP Range requests.
+                // The FileStream is seekable, so ASP.NET Core honours an incoming
+                // Range header by slicing the file and responding 206 Partial Content.
+                return File(innerStream, streamMimeType, enableRangeProcessing: true);
             }
 
             // Offset path: route through TrackContentService.GetAudioBinaryAsync (Track B's