feat(ui): Enable timeline scroll on speed graph

Adds a 'wheel' event listener to the speed graph container, binding it to the existing `handleTimelineWheel` function.

This change enhances user experience by allowing timeline scrubbing via the mouse scroll wheel when the cursor is over the speed graph. It creates consistent behavior across all major visualization components (radar canvas, timeline, and speed graph), improving the application's usability.

The logic is centralized within `initSyncUIHandlers` in `sync.js` to keep all synchronization-related event bindings in one place.

steps/src/sync.js

@@ -13,6 +13,7 @@ import {
   egoSpeedDisplay,
   canSpeedDisplay,
   autoOffsetIndicator,
+  speedGraphContainer
 } from "./dom.js";
 import { VIDEO_FPS } from "./constants.js";
 import { findRadarFrameIndexForTime, precomputeRadarVideoSync } from "./utils.js";
@@ -410,4 +411,8 @@ export function initSyncUIHandlers() {
   videoPlayer.addEventListener("wheel", handleVideoPanelWheel, {
     passive: false,
   });
+  // Use the speed graph container for radar frame seeking
+  speedGraphContainer.addEventListener("wheel", handleTimelineWheel, {
+    passive: false,
+  });
 }