fix: WaveformSeeker resize drift and mobile fast-tap crash

- Add ResizeObserver (JS observeResize/unobserveResize + C# OnWidthChanged)
  so _elementWidth stays current after window resize, fixing hover indicator drift
- Move _isSeeking = true before capturePointer await so a fast mobile tap
  that fires pointerup mid-await still commits the seek
- Replace all Duration!.Value null-forgiving dereferences with explicit
  Duration is > 0 guards in all four pointer event handlers
- Silence post-dispose resize callback rejections with .catch(() => {})

DeepDrftPublic.Client/wwwroot/js/waveformSeeker.js

@@ -10,3 +10,18 @@ export function capturePointer(element, pointerId) {
     // setPointerCapture keeps a drag tracking even when the pointer leaves the element bounds.
     element?.setPointerCapture?.(pointerId);
 }
+
+export function observeResize(element, dotNetRef) {
+    if (!element || typeof ResizeObserver === 'undefined') return null;
+    const observer = new ResizeObserver(entries => {
+        for (const entry of entries) {
+            dotNetRef.invokeMethodAsync('OnWidthChanged', entry.contentRect.width).catch(() => {});
+        }
+    });
+    observer.observe(element);
+    return observer;
+}
+
+export function unobserveResize(observer) {
+    observer?.disconnect();
+}