Replaces the deleted kit.ts + kit.css with DesignKit.tsx, gated behind
import.meta.env.DEV and loaded on ?kit. Shows the full tailwind.config.ts
token mapping (colours across all 6 palettes, typography, spacing,
radii, z-index) plus buttons, ActivityRail states, SessionCard states,
and terminal chrome, all switchable live through ThemeContext.
Closes#254
