Adds src/admin/pages/UiKit.tsx and registers a DEV-only <Route path="ui-kit">
sibling to <Route path="login">, outside the AdminLayout shell. The lazy import
uses the ternary `import.meta.env.DEV ? lazy(...) : null` pattern so Vite/Rolldown
evaluates the condition at build time and the UiKit chunk is entirely absent from
production bundles (confirmed: no UiKit-*.js in dist-client/assets after build:client).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
