fix(seo): Wave 1 — fix critical SEO gaps

- Fix duplicate faq/ruFaq keys in dice.json, list.json, colors.json
- Add <main> landmarks to about/privacy pages (EN + RU)
- Move YandexRTB after <main> on homepage
- Add x-default hreflang to BaseLayout
- Configure sitemap i18n with hreflang alternates
- Add Open Graph + Twitter Card meta tags
- Create default OG image (1200x630)

.sisyphus/evidence/task-3-ad.txt

@@ -0,0 +1,8 @@
+TASK: Fix ad placement on homepage
+FILE: src/pages/index.astro
+CHANGE: Moved <YandexRTB /> from before <main> to after </main>
+BUILD: PASS (npm run build exited 0)
+VERIFICATION:
+  - dist/index.html: </main> at byte 43284
+  - dist/index.html: yandex_rtb at byte 43335
+  - Order correct: True (main content before ad block)