fix(timeline): move getPlaybackTimeMs ref sync out of render phase

Per CodeRabbit review on #120: PlaybackCursor wrote to
getPlaybackTimeMsRef.current directly in the render body instead of in an
effect. Writing to a ref during render is a React anti-pattern — it can leave
the ref holding a stale value from a discarded render pass under concurrent
rendering. Moved the assignment into a useEffect keyed on getPlaybackTimeMs,
matching the recommended pattern for "latest callback" refs.

Verified: tsc --noEmit clean, biome clean, rafCoalescer.test.ts and
zoomRegionUtils.test.ts pass (7/7).

Author: EtienneLescot

src/components/video-editor/timeline/TimelineEditor.tsx

@@ -316,7 +316,9 @@ function PlaybackCursor({
 	// ancestor's (much heavier) re-render is in flight — see issue #111.
 	const [liveTimeMs, setLiveTimeMs] = useState(currentTimeMs);
 	const getPlaybackTimeMsRef = useRef(getPlaybackTimeMs);
-	getPlaybackTimeMsRef.current = getPlaybackTimeMs;
+	useEffect(() => {
+		getPlaybackTimeMsRef.current = getPlaybackTimeMs;
+	}, [getPlaybackTimeMs]);
 
 	useEffect(() => {
 		if (!getPlaybackTimeMsRef.current) {