0.4.0: clipart drawer, animation paths, bezier keyframes

- Clipart library: new left-edge drawer (5px idle, expands on hover,
  400px when open). User picks a local folder via the File System Access
  API in settings; directory handle is persisted in IndexedDB so the
  library survives reloads. Dragging a thumbnail onto the stage creates
  an image asset + layer at the drop point.
- Animation paths on the stage: selecting a keyframe draws the tween
  path from the previous keyframe to the selected one as a solid
  polyline with per-frame draggable dots. All other segments on the
  same position-track render as dashed ghost lines so the full curve
  progression is visible.
- Path dots grow on hover (2.5 → 6 px), show a grab cursor, and have a
  wider hit area so they're easier to grab.
- Bezier-curved position interpolation: keyframes gain optional
  ctrlX/ctrlY fields. When set, x/y tween along a quadratic bezier
  through the control point instead of a straight line. Dragging a
  path dot solves for the control so the curve passes through the
  drag point at its frame progress. Mirrored across all four render
  pipelines.
- Tests: new suites for polygon actions, freeform drawing lifecycle,
  polygon + stroke rendering (15 new tests). Fixed the mask-render
  tests that relied on a default-transparent canvas background.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -6,3 +6,6 @@ dist-ssr
 
 .DS_Store
 *.log
+
+# Local clipart working folder
+/clipart/