refactor: use shared nav header on demo page

Replace the bespoke header-top/tagline/version-badge layout with the same nav bar used on the landing page: muse · Demo (active) · Domain Registry · Plugin Guide · v{VERSION}

Remove now-dead CSS: .header-top, header h1, .tagline, .header-nav-link, .version-badge. Trim stats-bar top padding to 16px (nav provides visual separation). The v{{VERSION}} token in the nav is still live-interpolated from the build data.