fix(overlay): visible + a11y feedback for keyboard selection (#17)

Selection class had no CSS rule — keyboard nav worked but the user
couldn't see which row was active. Wire .selected to --sel token in
both themes, layering on top of :hover so cursor-parked highlight
wins when needed.

Add aria-selected on rows + aria-activedescendant on the listbox
so screen readers can follow the keyboard cursor. Listbox gets
aria-label and tabindex=-1 (focusable on programmatic .focus()
without being tab-stop).

No rust changes; tray-show path already routes through Show ->
focus_input at host.rs:692.