Feature Request:

Add support for rendering shadow indicators at the edges (top/bottom/left/right) of scrollable containers to visually hint scrollability.

Requirements:

1. Implement shadow rendering in web_content.frag.
2. Each scroll container (Instance) should support configurable shadow center height and color.
3. Shadow height should decrease from the center towards both sides.
4. When the scroll is near the edge, the shadow height should gradually decrease to zero, with a configurable maximum height.
5. Shadow height should be proportional to the element's width/height (default max height = 20%).

This will help users identify which edges of a scrollable container can be scrolled, improving UX.