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-2-main.txt

@@ -0,0 +1,16 @@
+Task: Fix <main> landmarks on about/privacy pages
+
+Files modified:
+- src/pages/about.astro
+- src/pages/ru/about.astro
+- src/pages/privacy.astro
+- src/pages/ru/privacy.astro
+
+Verification:
+- npm run lint: 0 errors
+- npm run build: success
+- grep dist for <main> on about/privacy pages: all 4 files contain <main> tag
+
+Pattern applied:
+- Breadcrumb nav and page header remain outside <main>
+- Content sections wrapped in <main> following GeneratorLayout.astro pattern