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.

.cursor/commands/review.md

@@ -37,6 +37,7 @@ Review every point below carefully to ensure files follow consistent code style
 - [ ] Each file has one single purpose/responsibility
   - Example: `timeline/index.tsx` should not define `validateElementTrackCompatibility` — that belongs in a lib file
   - Example: `lib/timeline-utils.ts` should not declare `TRACK_COLORS` — that belongs in `constants/`
+- [ ] File name accurately reflects what the file contains — a misleading name is a bug waiting to happen
 - [ ] Business logic lives in either `src/lib`, `src/core` or `src/services` folder
 
 ## Comments
@@ -134,7 +135,7 @@ Do NOT review by reading the file top-to-bottom and noting what jumps out. Inste
 4. After all sections are checked, do a final pass: re-read every checklist item and confirm you didn't skip it
 
 Before outputting the table, list each checklist section and confirm you checked it:
-`Signatures ✓ | TypeScript ✓ | JSX ✓ | Organization ✓ | Comments ✓ | Naming ✓ | Tailwind ✓ | State ✓ | Quality ✓ | Keywords ✓`
+`Signatures ✓ | TypeScript ✓ | JSX ✓ | Organization ✓ | File Names ✓ | Comments ✓ | Naming ✓ | Tailwind ✓ | State ✓ | Quality ✓ | Keywords ✓`
 
 ---
 

.cursor/rules/ultracite.mdc

@@ -28,7 +28,6 @@ Ultracite enforces strict type safety, accessibility standards, and consistent c
 ### Accessibility (a11y)
 
 - Always include a `title` element for icons unless there's text beside the icon.
-- Always include a `type` attribute for button elements.
 - Accompany `onClick` with at least one of: `onKeyUp`, `onKeyDown`, or `onKeyPress`.
 - Accompany `onMouseOver`/`onMouseOut` with `onFocus`/`onBlur`.