checks user sign in before allowing buying/bidding

frontend/script.js

@@ -0,0 +1,15 @@
+let USER_SIGNED_IN = false;
+
+function check_sign_in(game) {
+    const is_user_signed_in = localStorage.getItem("USER_SIGNED_IN");
+
+    if (is_user_signed_in !== null && is_user_signed_in === 'true') {
+        window.location.href = "./buy.html?game=" + game;
+    } else {
+        window.location.href = "./signin.html";
+    }
+}
+
+function set_user_signed_in() {
+    localStorage.setItem('USER_SIGNED_IN', true);
+}