theme-toggle (#52)

* default to system preferred theme

* theme svg in footer

* accounted for undefined window

app/[lang]/components/Footer.tsx

@@ -1,6 +1,7 @@
 import Link from "next-intl/link";
 import { FaGithub, FaRegEnvelope } from "react-icons/fa";
 import { useTranslations } from "next-intl";
+import ThemeSwitcher from "@/app/[lang]/components/ThemeSwitcher";
 
 export default function Footer() {
   const t = useTranslations("Footer");
@@ -34,6 +35,9 @@ export default function Footer() {
             EN
           </Link>
         </div>
+        <div className="text-xs">
+          <ThemeSwitcher />
+        </div>
       </div>
     </footer>
   );