feat: clarify timeline selection states

src/client/app.ts

@@ -770,12 +770,14 @@ function updateTimelineBrushOverlay(startX, endX) {
 
   const leftX = clamp(Math.min(startX, endX), 0, width);
   const rightX = clamp(Math.max(startX, endX), 0, width);
+  timelineChart.classList.add("selecting");
   timelineBrush.classList.add("visible");
   timelineBrush.style.left = leftX + "px";
   timelineBrush.style.width = Math.max(1, rightX - leftX) + "px";
 }
 
 function clearTimelineBrushOverlay() {
+  timelineChart.classList.remove("selecting");
   timelineBrush.classList.remove("visible");
   timelineBrush.style.left = "0";
   timelineBrush.style.width = "0";