Migrate from frame-centric to AnimationData system

Replaces legacy Frame-based object positioning and shape management with
AnimationData curves throughout the codebase. This enables time-based
animation instead of discrete frame indices, providing smoother playback
and more flexible keyframe editing.

Key changes:
- Remove currentFrame getter and frame.keys lookups
- Replace setFrameNum() with setTime() for continuous time navigation
- Add Layer.addShape()/removeShape() with AnimationData integration
- Migrate actions (move, group, delete, z-order) to use animation curves
- Update keyboard shortcuts and drag operations to modify curves directly
- Leave "holes" in shapeIndex values for proper undo/redo support

Rendering now fully driven by AnimationData curves (exists, zOrder,
shapeIndex for shapes; x, y, rotation, scale for objects).

src/utils.js

@@ -177,7 +177,8 @@ function floodFillRegion(
   fileHeight,
   context,
   debugPoints,
-  debugPaintbucket) {
+  debugPaintbucket,
+  shapes) {
 
   let halfEpsilon = epsilon/2
 
@@ -201,8 +202,6 @@ function floodFillRegion(
     }
     return false;
   }
-
-  const shapes = context.activeObject.currentFrame.shapes;
   const visited = new Set();
   const stack = [startPoint];
   const regionPoints = [];