fix(webapp): clear isAnimating when a resize drag interrupts a collapse

Grabbing the resize handle cancels an in-flight collapse/expand animation
frame but left the isAnimating flag set. A drag that then rests via
writeVisual (width >= DEFAULT_WIDTH) never reset it, so the side menu kept
hiding its scrollbar gutter until the next full collapse/expand. Clear the
flag on handle pointer-down so the reserved gutter returns on release.

Author: samejr

apps/webapp/app/components/navigation/SideMenu.tsx

@@ -519,6 +519,9 @@ export function SideMenu({
         cancelAnimationFrame(rafRef.current);
         rafRef.current = null;
       }
+      // Grabbing the handle interrupts any in-flight collapse/expand; clear the flag so a drag that
+      // rests via writeVisual (not animateTo) doesn't strand it true and keep the gutter hidden.
+      setIsAnimating(false);
       // Never allow two concurrent drags.
       dragCleanupRef.current?.();