[ScrollArea] Add scrollbar customization and expose sub-component refs

Summary

ScrollArea wraps Base UI's ScrollArea primitive. Lacks configuration props for scrollbar appearance and missing accessibility attributes.

Improvements

3. Make overscroll-behavior configurable

Hardcoded to auto in viewport CSS. Should be a prop for controlling scroll chaining behavior.

4. Expose sub-component refs via compound pattern

Cannot access Viewport or Content refs. Cannot customize Content/Viewport classNames from parent.

5. Add corner element customization

Corner element is rendered but not customizable.

API Differences from Base UI

Moved to Cross-Cutting Issues

The following items have been moved to cross-cutting issues that address them across all components:

• Replace hardcoded CSS values with design tokens (#672)
• Accessibility baseline (#673)

Files

• packages/raystack/raystack/components/scroll-area/

Start by reading the files under packages/raystack/raystack/components/scroll-area/ and compare the existing wrapper with the Base UI ScrollArea primitive. Trace the current Viewport, Content, Scrollbar, Thumb, and Corner usage, then verify that the requested props, refs, classNames, and corner customization are exposed without duplicating the cross-cutting work moved to issues #672 and #673.