Replace MudSlider seekbar with WaveformSeeker loudness-waveform control

DOM bar chart with clip-overlay progress split; pointer-capture drag;
WaveformProfile fetched on load (fire-and-forget, cancellable); flat
fallback when no profile; small lazily-loaded waveformSeeker.js for
getBoundingClientRect and setPointerCapture.

DeepDrftPublic.Client/Services/AudioPlayerService.cs

@@ -33,6 +33,9 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
     /// </summary>
     public TrackDto? CurrentTrack { get; protected set; }
 
+    /// <inheritdoc />
+    public double[]? WaveformProfile { get; protected set; }
+
     // Events
     public EventCallback? OnStateChanged { get; set; }
     public EventCallback? OnTrackSelected { get; set; }