investigate(issue-17): keyboard navigation gap report

Issue body is terse ('cannot navigate with keyboard'). Investigation
finds keyboard handlers exist (app.js:644-711) and selection state
moves correctly, but no CSS rule for .selected class — user can't
see which row is active. Classify as visual-feedback bug, not a
missing handler. Also flag tray-show focus loss and missing a11y
attrs (role=listbox/option present but no aria-selected or
aria-activedescendant).

Scope: S — style.css + minor app.js + tray.rs focus call.