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/main.js

@@ -11,10 +11,11 @@
 
     /* ------------------------------------------------------- the language */
 
+    var titleEl = document.querySelector("title");
     var description = document.querySelector('meta[name="description"]');
-    var TITLE_EN = document.title;
-    var TITLE_RU = "CursorLang — раскладка клавиатуры у курсора";
-    var DESCRIPTION_EN = description.getAttribute("content");
+    var TITLE_EN = titleEl.getAttribute("data-title-en");
+    var TITLE_RU = titleEl.getAttribute("data-title-ru");
+    var DESCRIPTION_EN = description.getAttribute("data-content-en");
     var DESCRIPTION_RU = description.getAttribute("data-content-ru");
 
     function applyLanguage(lang) {
@@ -44,8 +45,15 @@
         if (!button) { return; }
 
         var lang = button.getAttribute("data-set-lang");
-        applyLanguage(lang);
         remember("cursorlang.lang", lang);
+
+        // EN and RU live on separate URLs now, for search engines to rank
+        // independently — switching language means a real navigation.
+        var onRuPage = location.pathname.indexOf("/ru") === 0;
+        if (lang === "ru" && !onRuPage) { location.href = "/ru/"; return; }
+        if (lang === "en" && onRuPage) { location.href = "/"; return; }
+
+        applyLanguage(lang);
     });
 
     /* ---------------------------------------------------------- the theme */