fix(seo): pass lang as a prop to Header/LanguageSwitcher instead of useParams

Since the vite-react-ssg migration replaced the dynamic /:lang route param
with literal /fr and /en routes, useParams().lang always returned undefined
here, breaking the header brand link (linked to /undefined/) and silently
breaking the language switcher (always resolved otherLang to 'fr').

frontend/src/components/LangLayout.jsx

@@ -17,7 +17,7 @@ export function LangLayout({ lang }) {
 
   return (
     <>
-      <Header theme={theme} onToggleTheme={toggleTheme} />
+      <Header lang={lang} theme={theme} onToggleTheme={toggleTheme} />
       <main>
         <Outlet />
       </main>