Refactor `VersionSelectionOverlay` style for accessibility

[RRosio]

The VersionSelectionOverlay style makes the HTMLSelect completely transparent (opacity: 0) and positioned absolutely over the custom-styled version display. This creates an accessibility issue because screen reader users won't be able to see the current selection, and keyboard users might be confused about what they're interacting with since the visual feedback doesn't match the actual select element.

Consider adding proper ARIA attributes or using a custom accessible dropdown component that maintains visual and programmatic consistency.

Originally posted by @Copilot in https://github.com/mamba-org/gator/pull/328#discussion_r2612655725