Platform Aware Keys

A client component: it learns the system only after hydration, so the file carries "use client". The server does not know the client's system, and guessing it at render time desynchronises the markup — until detection finishes, the Ctrl variant is shown, which is understood on any system, unlike Ctrl+. The platform prop turns automatic detection off and pins a variant for documentation. The width of the modifier key is increased for Ctrl by the data-system attribute: the glyph is wider than a letter, and without the correction the row of keys looks uneven. The theme comes from the surrounding color-scheme through light-dark(): the component has no background by default, it sits directly on the page background and darkens with it, and a panel is switched on by the background prop.