Floating action bar for listNav selections — Set/Array/number, flat actions
Click rows to toggle their selection. The bar fades in from the bottom when the selection set is non-empty. Wired up in real apps to `listNav`'s `selection`. Flat actions take a single-key hotkey (`key: 'a'` etc.) — press it while the bar is visible and not typing to fire the action; the key renders as a hint on the button.