Add technical SEO foundation, MK/SI about-us + timeline, content fixes

SEO (all 201 pages, en/mk/si):
- Inject canonical, hreflang (en/mk/sl + x-default), Open Graph, Twitter
  cards and Organization JSON-LD via idempotent SEO:START/END block
- Generate multilingual sitemap.xml (xhtml:link alternates) + robots.txt
- Curated meta descriptions for lead-less pages (home, become-a-member,
  faq, news hub, my-route); every page now has a description
- Absolute URLs target final domain msosorg.com, not the temp host
- Add reusable tools/seo_inject.py (+ README)

Content fixes:
- Translate about-us and timeline-and-milestones into MK + SI, wire up
  their pages, and repair the previously-dead "Who We Are" nav links
  across all mk/si pages
- Remove duplicate en/projects/makedonska-studentska-vecer-vo-ljubljana
  (unreferenced; already covered by macedonian-student-night-in-ljubljana)

Locale correctness:
- Normalise Slovene pages to <html lang="sl"> (correct ISO 639-1)
- Map sl->si in main.js so the language-switcher highlight and newsletter
  i18n keep working on Slovene pages (data-lang="si" still maps to /si/)

MK/SI translations and descriptions are AI-drafted; flag for native review.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://msosorg.com/sitemap.xml