Writes a shortcut once, for every platform: keys joined by "+", with
"Mod" for Command on a Mac and Ctrl elsewhere. The hint carries both
forms and blockr.ui shows the one that applies (design system, "Keyboard
shortcuts"): "Mod+Shift+S" reads ⌘⇧S on a Mac and Ctrl+Shift+S
elsewhere, and "Mod+Enter" reads ⌘↵ or Ctrl+↵. Put it in a menu row's
meta slot (menu_item()) or after a button's label.
Value
A <span>, with controls_dep() attached.