Add 9 new shared s-expression components (cart-mini, auth-menu, account-nav-item, calendar-entry-nav, calendar-link-nav, market-link-nav, post-card, base-shell, error-page) and wire them into all fragment route handlers. 404/403 error pages now render entirely via s-expressions as a full-page proof-of-concept, with Jinja fallback on failure. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>