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/wwwroot/js/waveformSeeker.js

@@ -0,0 +1,12 @@
+// Geometry helpers for WaveformSeeker. Deliberately tiny and standalone — this is NOT part of
+// the TypeScript audio bundle (DeepDrftPublic/Interop/audio → wwwroot/js/audio). Loaded lazily as
+// an ES module by the component via IJSObjectReference, so it adds no global script tag.
+
+export function getWidth(element) {
+    return element ? element.getBoundingClientRect().width : 0;
+}
+
+export function capturePointer(element, pointerId) {
+    // setPointerCapture keeps a drag tracking even when the pointer leaves the element bounds.
+    element?.setPointerCapture?.(pointerId);
+}