Add blog via Astro content collections

Introduce blog support: content collection schema, listing and post
routes, and a sample Markdown post. Update docs and TODO; add blog
assets dir and adjust color variables in docs. Also set
absolute_redirect off in nginx.conf for container routing.

TODO.md

@@ -9,6 +9,7 @@
 ✅ Sitemap integration
 ✅ robots.txt
 ✅ Accessibility improvements (ARIA labels, semantic HTML)
+✅ Blog with Astro Content Collections (Markdown, no CMS)
 
 ## Immediate Next Steps
 
@@ -28,11 +29,11 @@
 - [ ] Consider adding subtle background patterns or textures
 
 #### Medium Term
-- [ ] Create a blog section using Astro Content Collections
+- [x] Create a blog section using Astro Content Collections
+- [ ] Add RSS feed for blog
 - [ ] Add a projects page that pulls from Gitea API
 - [ ] Create reusable components for repeated elements
 - [ ] Add a contact form (self-hosted solution)
-- [ ] Add RSS feed if blog is implemented
 
 #### Long Term
 - [ ] Theme toggle (dark/light, or alternate color schemes)
@@ -45,7 +46,7 @@
 
 - Network is required for npm install (packages not included in repo)
 - Gitea registry requires authentication (document login process)
-- No CMS - content updates require code changes (intentional for now)
+- No CMS - blog posts are Markdown files committed to the repo (intentional — no attack surface)
 
 ## Deployment Checklist