Add scroll fade indicators to Modal content area.

[bendansby]

Context

When the Modal's content overflows the visible area, there was no visual indication that more content existed beyond the top or bottom edge. This adds a soft fade above/below the scrollable region that appears only when there's content scrolled out of view in that direction.

🔧 What changes

• Adds the classic Lea Verou scroll-shadow four-gradient pattern to the Modal content area:
  • Two background-attachment: local white "covers" at top and bottom that scroll with the content. They sit over the static shadows when you're at the edge, hiding them.
  • Two background-attachment: scroll dark gradients that stay fixed to the viewport. As soon as content scrolls past an edge, the cover moves out of the way and the shadow is revealed.
• White covers are 48px tall and reach full opacity at 50% of their height, so they fully hide the 32px dark shadow when at an edge (no faint shadow bleeding through).
• Listed in the V12 patch changelog.

Component completion checklist

• Changes are clearly documented (V12 changelog updated)
• No API change — purely a CSS addition
• No new major version (patch-level change to V12)
• Component Catalog example unchanged — the existing Modal example with long content already exercises this
• Reviewers
  • Component library owner
  • Designer (since this is a user-visible change)

Notes for reviewers

• Pure CSS, no JS / scroll listeners — uses background-attachment: local vs scroll to detect overflow direction.
• Works on all modern browsers (FF, Safari, Chrome, Edge).
• The fade is visible against the white modal background — this assumes the content area stays white, which is currently always the case.