Task 14: Create RU blog post page src/pages/ru/blog/[slug].astro

Build result: PASS
Pages built: 77
RU blog post page: compiled successfully (no RU blog posts yet, so no static pages generated)
EN blog post page: /blog/test-post/index.html generated

Files created/modified:
- src/pages/ru/blog/[slug].astro (created)
- src/layouts/BlogLayout.astro (fixed locale-aware prev/next links)

Verification:
- npm run build exited with code 0
- No TypeScript or Astro errors
- RU [slug].astro correctly filters getCollection("blog") by lang === "ru"
- BlogLayout prev/next navigation updated to use locale-aware paths (/ru/blog/ for RU, /blog/ for EN)