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>

src/lib/interpolate.ts

@@ -105,7 +105,16 @@ function sampleKeyframeSet(
       }
       const raw = (time - a.time) / (b.time - a.time)
       const t = easings[b.easing](raw)
-      return lerpProps(merge(base, a.props), merge(base, b.props), t)
+      const pa = merge(base, a.props)
+      const pb = merge(base, b.props)
+      const out = lerpProps(pa, pb, t)
+      // Bezier-curved position: if b defines a quadratic control, override x/y
+      if (b.ctrlX !== undefined && b.ctrlY !== undefined) {
+        const oneMinus = 1 - t
+        out.x = oneMinus * oneMinus * pa.x + 2 * oneMinus * t * b.ctrlX + t * t * pb.x
+        out.y = oneMinus * oneMinus * pa.y + 2 * oneMinus * t * b.ctrlY + t * t * pb.y
+      }
+      return out
     }
   }
   return { ...base }