Implement user authentication and registration; enhance error handling and database interaction in API

public/templates/setup/setup.html

@@ -100,7 +100,8 @@
                 method: "POST",
                 body: JSON.stringify({
                     username: username,
-                    password: password
+                    password: password,
+                    password_repeat: password_repeat
                 }),
                 headers: {
                     "Content-type": "application/json; charset=UTF-8"