Add disconnection and minor improvements on login page.

frontend/src/app/pages/disconnection/disconnection.component.html

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+<h1>Disconnection...</h1>
+<mat-spinner></mat-spinner>

frontend/src/app/pages/disconnection/disconnection.component.scss

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+:host {
+    display: flex;
+    flex-direction: column;
+    justify-content: center;
+    align-items: center;
+}

frontend/src/app/pages/disconnection/disconnection.component.ts

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+import { Component, OnInit, inject } from '@angular/core';
+import {MatProgressSpinnerModule} from '@angular/material/progress-spinner'; 
+import { AuthenticationService } from '../../core/service/authentication.service';
+import { Router } from '@angular/router';
+
+@Component({
+  selector: 'app-disconnection',
+  standalone: true,
+  imports: [MatProgressSpinnerModule],
+  templateUrl: './disconnection.component.html',
+  styleUrl: './disconnection.component.scss'
+})
+export class DisconnectionComponent implements OnInit {
+  private authenticationService = inject(AuthenticationService);
+  private router = inject(Router);
+
+  ngOnInit(): void {
+    this.authenticationService.unauthenticate();
+    this.router.navigate(['/home']);
+  }
+}