XDSScrollArea — other internal library migration (940 usages)

## other internal library → XDS Migration Gap

**other internal library usage count:** 940

### What it does
Custom scrollbar styling container. other internal library uses `ScrollArea` heavily for panels, code blocks, chat views, and any content area that needs styled scrollbars. It provides a consistent cross-browser scrollbar appearance with custom track/thumb styling.

### Prior art in XDS
- #171 (XDSScrollableArea) — closed, not implemented

### Approach considerations
1. **Wrap Radix scroll-area** — Radix provides a well-tested accessible scroll container. XDS could wrap `@radix-ui/react-scroll-area` with XDS styling tokens.
2. **CSS-only approach** — Modern CSS `scrollbar-width`, `scrollbar-color`, and `::-webkit-scrollbar` pseudo-elements may be sufficient. Lighter weight, no JS runtime.
3. **Hybrid** — CSS-only by default with an optional JS-enhanced mode for custom thumb rendering.

### Key other internal library features to evaluate
- Vertical + horizontal scrolling
- Auto-hide scrollbars
- Custom scrollbar sizing
- Content overflow detection
- Composable with panels, dialogs, and sidebars

### Context
High-usage other internal library component (940 imports). Part of the other internal library → XDS OSS migration effort to ensure feature parity for teams migrating.