implemented user login.

next: find a solution for routing

frontend/src/Components/Menu.svelte

@@ -3,11 +3,13 @@
     import { user } from '../user.svelte.js';
 
     function logout(){
-        user.username = null;
+        user.name = null;
     }
 </script>
 <nav>
     <a href="/">{t('nav.Home')}</a>
     <a href="https://svelte.dev/tutorial/svelte/state" target="_blank">{t('nav.Tutorial')}</a>
+    {#if user.name }
     <a href="#" on:click={logout}>Logout</a>
+    {/if}
 </nav>