feat(blog): add author pages and author frontmatter (#124)

[Escelit]

Closes #124

Summary

Implements blog author pages (#124): maps MDX author frontmatter to src/data/authors.json, adds a /blog/author/:id route, links bylines to author pages, and collapses opted-out authors to "Wraith Team".

Changes

• src/utils/blog.ts: Author/AuthorProfile types, getAuthorById, resolveAuthor, getPostsByAuthor; each post carries authorName/authorId.
• src/pages/Blog.tsx: bylines link to /blog/author/:id for opted-in authors; new BlogAuthor page (avatar/initials, bio, links, post list) with graceful "Author Not Found" fallback for unknown ids.
• src/App.tsx: /blog/author/:authorId route.
• src/data/authors.json + authors-optout.json: seed authors (team opted-out, one opted-in contributor).
• scripts/sitemap.ts: opt-in author routes.
• scripts/gen-rss.mjs: resolve author ids to display names in the feed.
• Added a second MDX post to exercise the author listing; updated docs and tests.

Acceptance

• /blog/author/:id works for every opted-in author
• Every post links its author byline to the author page (or "Wraith Team" if opted out)
• Unknown author ids fall back gracefully (no 500, no crash)
• Build passes (format + build hooks green, 14 routes; blog + rss tests green)
• Sitemap/feed include author data (SEO)

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[truthixify]

Thanks @Escelit. Worth coordinating this one, because you have three open PRs here (#137, #138, #140) and all three edit src/pages/Blog.tsx, so they will keep conflicting with each other no matter what order they land in.

Suggested order, rebasing each on the previous once it merges:

1. feat(blog): add author pages and author frontmatter (#124) #138 (this one, author pages) first, since it changes src/utils/blog.ts frontmatter parsing that the others build on
2. feat(blog): add tag archive pages, related posts, and per-tag RSS feeds #137 (tag archives, related posts, per-tag RSS)
3. feat(seo): expand structured data with shared jsonld helpers (#126) #140 (structured data) last, because its BreadcrumbList covers the /blog/author/:id route that feat(blog): add author pages and author frontmatter (#124) #138 introduces

For this PR the conflicts are src/pages/Blog.tsx and src/utils/blog.ts against #136, which added reading-time and the TOC:

git fetch origin
git rebase origin/develop

Take the reading-time and TOC additions from develop and layer your author byline changes on top.