feat: make hover tooltip follow the cursor

Update hoverPoint on every mouse move while over a part, not just
when the hovered part changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Controls/PlateView.cs

@@ -364,8 +364,10 @@ namespace OpenNest.Controls
                 }
 
                 if (hitPart != hoveredPart)
-                {
                     hoveredPart = hitPart;
+
+                if (hoveredPart != null)
+                {
                     hoverPoint = e.Location;
                     Invalidate();
                 }