feat: update user retrieval and add new users table; enhance MainForm with user fetching and localization support

backend/scheme.sql

@@ -8,6 +8,14 @@ CREATE TABLE users (
     created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
 );
 
+CREATE TABLE usersNEW (
+    id INT AUTO_INCREMENT PRIMARY KEY,
+    username VARCHAR(100) NOT NULL UNIQUE,
+    first_name VARCHAR(100) NOT NULL,
+    last_name VARCHAR(100) NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
 -- Trigger zur automatischen Generierung eines 25‑stelligen Keys
 DELIMITER $