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/notepads/seo-blog-categories/learnings.md

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+## Task 6: Open Graph + Twitter Card Meta Tags
+
+- Added `ogImage?: string` prop to `BaseLayout.astro` Props interface.
+- Implemented full OG tag set: `og:site_name`, `og:locale`, `og:type`, `og:title`, `og:description`, `og:url`, `og:image`.
+- Implemented full Twitter Card tag set: `twitter:card` (summary_large_image), `twitter:title`, `twitter:description`, `twitter:image`.
+- Default OG image (`/og-default.jpg`) is used when `ogImage` prop is omitted.
+- `ogImageUrl` helper ensures absolute URLs by prefixing `https://randify.pro` to relative paths.
+- Default OG image generated via Node.js + sharp (1200x630, dark background with accent radial gradients and "Randify" branding).
+- Build passes; tags verified in `dist/index.html` and `dist/generators/dice/index.html`.
+