feat(api): refactor API client usage across frontend components

- Replaced direct axios calls with a centralized apiClient in Register, Login, and Activate components for improved maintainability and consistency.
- Updated backend base URL logic to support different environments, enhancing flexibility in API interactions.
- Added console logging in the authController for better tracking of user registration flow.

backend/controllers/authController.js

@@ -3,9 +3,12 @@ import { register, activateUser, login, logout } from '../services/authService.j
 const registerUser = async (req, res, next) => {
   try {
     const { email, password } = req.body;
+    console.log('registerUser', email, password);
     await register(email, password);
+    console.log('registerUser done');
     // Aus Sicherheitsgründen KEINE Userdaten (Passwort-Hash, Aktivierungscode, ...) zurückgeben
     res.status(201).json({ success: true });
+    console.log('registerUser response sent');
   } catch (error) {
     next(error);
   }