add route matcher to fix /login and /logout routes

Use SvelteKit param matcher to constrain [recipeLang] to only match
'recipes' or 'rezepte', preventing it from catching /login, /logout,
and other non-recipe routes.

src/routes/[recipeLang=recipeLang]/+layout.server.ts

@@ -0,0 +1,17 @@
+import type { LayoutServerLoad } from "./$types"
+import { error } from "@sveltejs/kit";
+
+export const load : LayoutServerLoad = async ({locals, params}) => {
+	// Validate recipeLang parameter
+	if (params.recipeLang !== 'rezepte' && params.recipeLang !== 'recipes') {
+		throw error(404, 'Not found');
+	}
+
+	const lang = params.recipeLang === 'recipes' ? 'en' : 'de';
+
+	return {
+		session: await locals.auth(),
+		lang,
+		recipeLang: params.recipeLang
+	}
+};