Add SEO basics: separate EN/RU pages, sitemap, OG banner

Splits the landing page into /  and /ru/ so each language can rank on
its own, with hreflang links between them; boot.js/main.js now do a
real navigation between the two instead of toggling text in place.
Adds robots.txt and sitemap.xml, JSON-LD SoftwareApplication markup,
Twitter Card tags, and a proper 1200x630 og-image.png in place of the
SVG favicon for social previews.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

site/sitemap.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
+        xmlns:xhtml="http://www.w3.org/1999/xhtml">
+    <url>
+        <loc>https://cursor-lang.alrakis.kz/</loc>
+        <xhtml:link rel="alternate" hreflang="en" href="https://cursor-lang.alrakis.kz/"/>
+        <xhtml:link rel="alternate" hreflang="ru" href="https://cursor-lang.alrakis.kz/ru/"/>
+        <xhtml:link rel="alternate" hreflang="x-default" href="https://cursor-lang.alrakis.kz/"/>
+        <changefreq>monthly</changefreq>
+        <priority>1.0</priority>
+    </url>
+    <url>
+        <loc>https://cursor-lang.alrakis.kz/ru/</loc>
+        <xhtml:link rel="alternate" hreflang="en" href="https://cursor-lang.alrakis.kz/"/>
+        <xhtml:link rel="alternate" hreflang="ru" href="https://cursor-lang.alrakis.kz/ru/"/>
+        <xhtml:link rel="alternate" hreflang="x-default" href="https://cursor-lang.alrakis.kz/"/>
+        <changefreq>monthly</changefreq>
+        <priority>1.0</priority>
+    </url>
+</urlset>