Improve dropdown menu accessibility state and focus

[web3blind]

Summary

This PR addresses part of #183 for DropdownMenu by improving the trigger state announcement and keyboard focus visibility.

Changes:

• add aria-haspopup="menu" and aria-expanded to a single trigger element when possible
• link the trigger to the menu with aria-controls while the menu is open
• add an explicit high-contrast outline for focused menu items, instead of relying only on a very subtle background change

Accessibility impact

Screen-reader users can now hear whether the dropdown trigger is expanded or collapsed, and keyboard users get a clearer focus indicator while navigating menu items.

This is intentionally a small, safe step toward the broader dropdown accessibility issue rather than a full rewrite of the component.

Validation

• corepack yarn lint — passed with existing warnings in unrelated preview components
• corepack yarn build — passed

Closes part of #183.