feat(seo): migrate to build-time prerendering with vite-react-ssg

Fixes SeoHead (title/canonical/hreflang/JSON-LD) to render during SSR via
vite-react-ssg's Head component instead of a client-only useEffect, so
crawlers see real tags without executing JS. Also fixes an SSR crash in
useTheme (localStorage access with no window during server render) and
removes now-duplicated static meta tags from index.html.

frontend/src/components/LangLayout.jsx

@@ -0,0 +1,27 @@
+import { useEffect } from 'react';
+import { Outlet } from 'react-router-dom';
+import i18n from '../i18n.js';
+import { Header } from './Header.jsx';
+import { Footer } from './Footer.jsx';
+import { useTheme } from '../hooks/useTheme.js';
+import '../styles/layout.css';
+
+export function LangLayout({ lang }) {
+  const { theme, toggleTheme } = useTheme();
+
+  useEffect(() => {
+    if (i18n.language !== lang) {
+      i18n.changeLanguage(lang);
+    }
+  }, [lang]);
+
+  return (
+    <>
+      <Header theme={theme} onToggleTheme={toggleTheme} />
+      <main>
+        <Outlet />
+      </main>
+      <Footer />
+    </>
+  );
+}