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.

4 files changed, 308 insertions, 0 deletions

diff --git a/apps/web/src/lib/commands/timeline/element/keyframes/index.ts b/apps/web/src/lib/commands/timeline/element/keyframes/index.ts
new file mode 100644
index 00000000..f221e33a
--- /dev/null
+++ b/apps/web/src/lib/commands/timeline/element/keyframes/index.ts
@@ -0,0 +1,3 @@
+export * from "./remove-keyframe";
+export * from "./retime-keyframe";
+export * from "./upsert-keyframe";
diff --git a/apps/web/src/lib/commands/timeline/element/keyframes/remove-keyframe.ts b/apps/web/src/lib/commands/timeline/element/keyframes/remove-keyframe.ts
new file mode 100644
index 00000000..f12f9829
--- /dev/null
+++ b/apps/web/src/lib/commands/timeline/element/keyframes/remove-keyframe.ts
@@ -0,0 +1,141 @@
+import { EditorCore } from "@/core";
+import {
+	getChannel,
+	getChannelValueAtTime,
+	getElementBaseValueForProperty,
+	removeElementKeyframe,
+	supportsAnimationProperty,
+	withElementBaseValueForProperty,
+} from "@/lib/animation";
+import { Command } from "@/lib/commands/base-command";
+import { updateElementInTracks } from "@/lib/timeline";
+import type { AnimationPropertyPath } from "@/types/animation";
+import type { TimelineElement, TimelineTrack } from "@/types/timeline";
+
+function sampleValueBeforeRemoval({
+	element,
+	propertyPath,
+	keyframeId,
+}: {
+	element: TimelineElement;
+	propertyPath: AnimationPropertyPath;
+	keyframeId: string;
+}): number | null {
+	const channel = getChannel({
+		animations: element.animations,
+		propertyPath,
+	});
+	const keyframe = channel?.keyframes.find(
+		(candidate) => candidate.id === keyframeId,
+	);
+	if (!channel || !keyframe) {
+		return null;
+	}
+
+	const baseValue = getElementBaseValueForProperty({ element, propertyPath });
+	if (baseValue === null || typeof baseValue !== "number") {
+		return null;
+	}
+
+	const sampled = getChannelValueAtTime({
+		channel,
+		time: keyframe.time,
+		fallbackValue: baseValue,
+	});
+	return typeof sampled === "number" ? sampled : null;
+}
+
+function removeKeyframeAndPersist({
+	element,
+	propertyPath,
+	keyframeId,
+}: {
+	element: TimelineElement;
+	propertyPath: AnimationPropertyPath;
+	keyframeId: string;
+}): TimelineElement {
+	const valueBefore = sampleValueBeforeRemoval({
+		element,
+		propertyPath,
+		keyframeId,
+	});
+
+	const nextAnimations = removeElementKeyframe({
+		animations: element.animations,
+		propertyPath,
+		keyframeId,
+	});
+
+	const isChannelNowEmpty =
+		getChannel({ animations: nextAnimations, propertyPath }) === undefined;
+	const shouldPersistToBase = isChannelNowEmpty && valueBefore !== null;
+
+	const baseElement = shouldPersistToBase
+		? withElementBaseValueForProperty({
+				element,
+				propertyPath,
+				value: valueBefore,
+			})
+		: element;
+
+	return { ...baseElement, animations: nextAnimations };
+}
+
+export class RemoveKeyframeCommand extends Command {
+	private savedState: TimelineTrack[] | null = null;
+	private readonly trackId: string;
+	private readonly elementId: string;
+	private readonly propertyPath: AnimationPropertyPath;
+	private readonly keyframeId: string;
+
+	constructor({
+		trackId,
+		elementId,
+		propertyPath,
+		keyframeId,
+	}: {
+		trackId: string;
+		elementId: string;
+		propertyPath: AnimationPropertyPath;
+		keyframeId: string;
+	}) {
+		super();
+		this.trackId = trackId;
+		this.elementId = elementId;
+		this.propertyPath = propertyPath;
+		this.keyframeId = keyframeId;
+	}
+
+	execute(): void {
+		const editor = EditorCore.getInstance();
+		this.savedState = editor.timeline.getTracks();
+
+		const updatedTracks = updateElementInTracks({
+			tracks: this.savedState,
+			trackId: this.trackId,
+			elementId: this.elementId,
+			elementPredicate: (element) =>
+				supportsAnimationProperty({
+					element,
+					propertyPath: this.propertyPath,
+				}),
+			update: (element) =>
+				removeKeyframeAndPersist({
+					element,
+					propertyPath: this.propertyPath,
+					keyframeId: this.keyframeId,
+				}),
+		});
+
+		editor.timeline.updateTracks(updatedTracks);
+	}
+
+	undo(): void {
+		if (!this.savedState) {
+			return;
+		}
+
+		const editor = EditorCore.getInstance();
+		editor.timeline.updateTracks(this.savedState);
+	}
+}
diff --git a/apps/web/src/lib/commands/timeline/element/keyframes/retime-keyframe.ts b/apps/web/src/lib/commands/timeline/element/keyframes/retime-keyframe.ts
new file mode 100644
index 00000000..29d95918
--- /dev/null
+++ b/apps/web/src/lib/commands/timeline/element/keyframes/retime-keyframe.ts
@@ -0,0 +1,75 @@
+import { EditorCore } from "@/core";
+import { retimeElementKeyframe, supportsAnimationProperty } from "@/lib/animation";
+import { Command } from "@/lib/commands/base-command";
+import { updateElementInTracks } from "@/lib/timeline";
+import type { AnimationPropertyPath } from "@/types/animation";
+import type { TimelineTrack } from "@/types/timeline";
+
+export class RetimeKeyframeCommand extends Command {
+	private savedState: TimelineTrack[] | null = null;
+	private readonly trackId: string;
+	private readonly elementId: string;
+	private readonly propertyPath: AnimationPropertyPath;
+	private readonly keyframeId: string;
+	private readonly nextTime: number;
+
+	constructor({
+		trackId,
+		elementId,
+		propertyPath,
+		keyframeId,
+		nextTime,
+	}: {
+		trackId: string;
+		elementId: string;
+		propertyPath: AnimationPropertyPath;
+		keyframeId: string;
+		nextTime: number;
+	}) {
+		super();
+		this.trackId = trackId;
+		this.elementId = elementId;
+		this.propertyPath = propertyPath;
+		this.keyframeId = keyframeId;
+		this.nextTime = nextTime;
+	}
+
+	execute(): void {
+		const editor = EditorCore.getInstance();
+		this.savedState = editor.timeline.getTracks();
+
+		const updatedTracks = updateElementInTracks({
+			tracks: this.savedState,
+			trackId: this.trackId,
+			elementId: this.elementId,
+			elementPredicate: (element) =>
+				supportsAnimationProperty({
+					element,
+					propertyPath: this.propertyPath,
+				}),
+			update: (element) => {
+				const boundedTime = Math.max(0, Math.min(this.nextTime, element.duration));
+				return {
+					...element,
+					animations: retimeElementKeyframe({
+						animations: element.animations,
+						propertyPath: this.propertyPath,
+						keyframeId: this.keyframeId,
+						time: boundedTime,
+					}),
+				};
+			},
+		});
+
+		editor.timeline.updateTracks(updatedTracks);
+	}
+
+	undo(): void {
+		if (!this.savedState) {
+			return;
+		}
+
+		const editor = EditorCore.getInstance();
+		editor.timeline.updateTracks(this.savedState);
+	}
+}
diff --git a/apps/web/src/lib/commands/timeline/element/keyframes/upsert-keyframe.ts b/apps/web/src/lib/commands/timeline/element/keyframes/upsert-keyframe.ts
new file mode 100644
index 00000000..370f6865
--- /dev/null
+++ b/apps/web/src/lib/commands/timeline/element/keyframes/upsert-keyframe.ts
@@ -0,0 +1,89 @@
+import { EditorCore } from "@/core";
+import { Command } from "@/lib/commands/base-command";
+import { supportsAnimationProperty, upsertElementKeyframe } from "@/lib/animation";
+import { updateElementInTracks } from "@/lib/timeline";
+import type { TimelineTrack } from "@/types/timeline";
+import type {
+	AnimationInterpolation,
+	AnimationPropertyPath,
+	AnimationValue,
+} from "@/types/animation";
+
+export class UpsertKeyframeCommand extends Command {
+	private savedState: TimelineTrack[] | null = null;
+	private readonly trackId: string;
+	private readonly elementId: string;
+	private readonly propertyPath: AnimationPropertyPath;
+	private readonly time: number;
+	private readonly value: AnimationValue;
+	private readonly interpolation: AnimationInterpolation | undefined;
+	private readonly keyframeId: string | undefined;
+
+	constructor({
+		trackId,
+		elementId,
+		propertyPath,
+		time,
+		value,
+		interpolation,
+		keyframeId,
+	}: {
+		trackId: string;
+		elementId: string;
+		propertyPath: AnimationPropertyPath;
+		time: number;
+		value: AnimationValue;
+		interpolation?: AnimationInterpolation;
+		keyframeId?: string;
+	}) {
+		super();
+		this.trackId = trackId;
+		this.elementId = elementId;
+		this.propertyPath = propertyPath;
+		this.time = time;
+		this.value = value;
+		this.interpolation = interpolation;
+		this.keyframeId = keyframeId;
+	}
+
+	execute(): void {
+		const editor = EditorCore.getInstance();
+		this.savedState = editor.timeline.getTracks();
+
+		const updatedTracks = updateElementInTracks({
+			tracks: this.savedState,
+			trackId: this.trackId,
+			elementId: this.elementId,
+			elementPredicate: (element) =>
+				supportsAnimationProperty({
+					element,
+					propertyPath: this.propertyPath,
+				}),
+			update: (element) => {
+				const boundedTime = Math.max(0, Math.min(this.time, element.duration));
+				return {
+					...element,
+					animations: upsertElementKeyframe({
+						animations: element.animations,
+						propertyPath: this.propertyPath,
+						time: boundedTime,
+						value: this.value,
+						interpolation: this.interpolation,
+						keyframeId: this.keyframeId,
+					}),
+				};
+			},
+		});
+
+		editor.timeline.updateTracks(updatedTracks);
+	}
+
+	undo(): void {
+		if (!this.savedState) {
+			return;
+		}
+
+		const editor = EditorCore.getInstance();
+		editor.timeline.updateTracks(this.savedState);
+	}
+}