Add user-friendly auth/error pages and consolidate markdown docs under /docs

[MohammedAlkindi]

Motivation

• Provide clear, non-technical error pages for visitors (including friendly messaging when Google sign-in fails) instead of exposing raw server errors.
• Surface actionable guidance for maintainers when authentication is broken due to missing Supabase env vars (SUPABASE_URL, SUPABASE_ANON_KEY).
• Standardize visual style for all error pages to improve UX and maintainability.
• Consolidate scattered Markdown docs into a single docs/ location to follow the repository convention and make documentation easier to find.

Description

• Added a shared stylesheet public/error-pages.css and created root-level HTTP error pages public/400.html, public/401.html, public/403.html, public/404.html (updated), public/405.html, public/500.html, and public/501.html that use the new stylesheet for consistent presentation.
• Added public/auth-error.html which shows a friendly sign-in error page and contains JS that detects messages mentioning missing Supabase env vars and replaces the UI message with a maintainer hint for SUPABASE_URL and SUPABASE_ANON_KEY.
• Updated vercel.json to include a rewrite mapping /auth/error to the new /public/auth-error.html so auth failures can be routed to the friendly page.
• Moved all repository Markdown files (except the root README.md) into docs/markdown/** and updated README.md to document the consolidated convention.

Testing

• Ran ./scripts/build.sh, which is a no-op for this static site, and it completed successfully.
• Ran ./scripts/validate-links.sh and it completed without errors.
• Verified Markdown layout with find . -name '*.md' | sort to confirm only the root README.md remains outside docs.

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Codex Task

[vercel]

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Project	Deployment	Actions	Updated (UTC)
proof-x	Ready	Preview, Comment	Apr 7, 2026 5:17pm