feat(frontend): basic NyxID blog page with content preview

[ctkm-aelf]

Summary

Ship a minimal NyxID blog page on the marketing site that renders blog posts
fetched from an external CDN, with a preview mode for unpublished drafts.

Motivation

Marketing needs a place to publish posts (release notes, deep dives, launch
announcements) without redeploying the frontend on every edit. Hosting the
content on a CDN lets writers iterate independently of the app's deploy
cadence and gives us a clean separation between code and content.

Scope

• New /blog route with index (post list) and /blog/:slug (post detail)
• Posts fetched at request time from a CDN base URL
• ?preview=<token> query param to render unpublished drafts from a
  separate preview path on the same CDN
• CDN base URL and preview token configured via env vars
  (e.g. VITE_BLOG_CDN_URL, VITE_BLOG_PREVIEW_TOKEN) so deploys can swap
  CDN buckets without code changes
• Markdown rendering (frontmatter for title / date / author / slug)
• Basic styling consistent with DESIGN.md

Out of scope

• Authoring UI / CMS — content is committed to the CDN bucket out-of-band
• Comments, reactions, search
• RSS / sitemap (follow-up)
• Pagination beyond a simple "latest N" list

Acceptance criteria

• /blog lists posts pulled from the configured CDN
• /blog/:slug renders a post's markdown with frontmatter metadata
• Preview mode renders draft posts when ?preview=<token> matches the
  configured token
• CDN URL + preview token are env-driven (documented in docs/ENV.md)
• Page passes the DESIGN.md visual checklist

Dependencies

• Blocked-by: infra: provision CDN for company blog and open-source project assets #525 — needs the CDN bucket and base URL defined. Until
  then, point at a placeholder bucket via env.

Deadline

Thu 2026-04-30 (W18).

[ctkm-aelf]

completed by PR #604