CE: Add accessibility basics and polish error pages

[ericflo]

Summary

• Skip-to-content link, ARIA landmarks (role="navigation", <main>, role="contentinfo"), and :focus-visible purple ring outlines across page_shell, index.html, and pricing.html
• Polished error pages with contextual icons, colored border-left accents, and friendlier copy (error_page, no_db_html, rate_limit_response, Stripe config errors)
• Form labels already present and correct in auth forms (verified)

Why

Accessibility basics is an explicit project goal that has not been addressed yet. Error pages were the last screens with minimal styling.