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/lib/commands/timeline/clipboard/paste.ts

@@ -13,6 +13,7 @@ import {
 	isMainTrack,
 	enforceMainTrackStart,
 } from "@/lib/timeline/track-utils";
+import { cloneAnimations } from "@/lib/animation";
 
 export class PasteCommand extends Command {
 	private savedState: TimelineTrack[] | null = null;
@@ -176,6 +177,10 @@ function buildPastedElements({
 			...item.element,
 			id: newElementId,
 			startTime,
+			animations: cloneAnimations({
+				animations: item.element.animations,
+				shouldRegenerateKeyframeIds: true,
+			}),
 		} as TimelineElement);
 	}