feat: implement keyframe animation system

Add keyframe support for transform, opacity, and volume properties.
Includes animation engine (interpolation, mutations, resolvers), timeline
markers with selection/snapping, properties panel toggles, keyframe-aware
renderer, and full undo/redo command support.

Also refactor element command constructors to object params, extract
timeline pixel math to pixel-utils.ts, and update cursor rules.

apps/web/src/components/editor/panels/timeline/timeline-tick.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { TIMELINE_CONSTANTS } from "@/constants/timeline-constants";
+import { timelineTimeToSnappedPixels } from "@/lib/timeline";
 import { formatRulerLabel } from "@/lib/timeline/ruler-utils";
 
 interface TimelineTickProps {
@@ -16,7 +16,7 @@ export function TimelineTick({
 	fps,
 	showLabel,
 }: TimelineTickProps) {
-	const leftPosition = time * TIMELINE_CONSTANTS.PIXELS_PER_SECOND * zoomLevel;
+	const leftPosition = timelineTimeToSnappedPixels({ time, zoomLevel });
 
 	if (showLabel) {
 		const label = formatRulerLabel({ timeInSeconds: time, fps });