fix: read live video.currentTime for rapid frame steps and add JSDoc

- Read currentTime directly from the video element instead of the React
  ref so rapid arrow key presses each advance by exactly one frame
- Add JSDoc docstrings to frameStep.ts exports

src/components/video-editor/VideoEditor.tsx

@@ -949,13 +949,17 @@ export default function VideoEditor() {
 					return;
 				}
 				e.preventDefault();
+				const video = videoPlaybackRef.current?.video;
+				if (!video) {
+					return;
+				}
 				const direction = e.key === "ArrowLeft" ? "backward" : "forward";
 				const newTime = computeFrameStepTime(
-					currentTimeRef.current,
-					durationRef.current,
+					video.currentTime,
+					Number.isFinite(video.duration) ? video.duration : durationRef.current,
 					direction,
 				);
-				handleSeek(newTime);
+				video.currentTime = newTime;
 				return;
 			}
 

src/lib/frameStep.ts

@@ -1,5 +1,10 @@
+/** Duration of a single frame in seconds at 60 FPS (~16.67ms). */
 export const FRAME_DURATION_SEC = 1 / 60;
 
+/**
+ * Compute the new playhead time after stepping one frame forward or backward.
+ * The result is clamped to the range [0, duration].
+ */
 export function computeFrameStepTime(
 	currentTime: number,
 	duration: number,