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>

server/render.html

@@ -80,7 +80,15 @@
         }
         var raw=(time-a.time)/(b.time-a.time);
         var t=easing(b.easing,raw);
-        return lerpProps(merge(layer.base,a.props), merge(layer.base,b.props), t);
+        var pa=merge(layer.base,a.props);
+        var pb=merge(layer.base,b.props);
+        var outS=lerpProps(pa,pb,t);
+        if(b.ctrlX!=null && b.ctrlY!=null){
+          var om=1-t;
+          outS.x=om*om*pa.x+2*om*t*b.ctrlX+t*t*pb.x;
+          outS.y=om*om*pa.y+2*om*t*b.ctrlY+t*t*pb.y;
+        }
+        return outS;
       }
     }
     return merge(layer.base);