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/Controls/AudioPlayerBar/AudioPlayerBar.razor.cs

@@ -29,12 +29,6 @@ public partial class AudioPlayerBar : ComponentBase, IAsyncDisposable
     /// </summary>
     private double DisplayTime => _isSeeking ? _seekPosition : (PlayerService?.CurrentTime ?? 0);
 
-    /// <summary>
-    /// Seek is enabled once track is loaded AND duration is known (from WAV header).
-    /// This allows seeking even during streaming, including seeking beyond buffered content.
-    /// </summary>
-    private bool CanSeek => IsLoaded && Duration.HasValue && Duration.Value > 0;
-
     protected override void OnParametersSet()
     {
         // PlayerService is cascaded by AudioPlayerProvider; once it arrives,