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/IPlayerService.cs

@@ -19,6 +19,14 @@ public interface IPlayerService
     string? ErrorMessage { get; }
     TrackDto? CurrentTrack { get; }
 
+    /// <summary>
+    /// Normalized loudness profile for the current track, each value in [0, 1], or null when no
+    /// profile is available (no track loaded, or the track has no stored profile). The seek zone
+    /// renders this as a waveform; a null profile drives the flat-but-seekable fallback. Fetched on
+    /// track select and cleared on unload/stop; <see cref="StateChanged"/> fires once it arrives.
+    /// </summary>
+    double[]? WaveformProfile { get; }
+
     // Events for UI updates
     EventCallback? OnStateChanged { get; set; }
     EventCallback? OnTrackSelected { get; set; }