Extend the web application to include user authentication, allowing registered users to log in and access personalized content on subsequent visits.

<div>
    <h3>Login</h3>
    <form [formGroup]="loginForm" (ngSubmit)="onSubmit()">
      <div class="form-group">
        <label for="username">Username</label>
        <input type="text" formControlName="username" class="form-control" [ngClass]="{ 'is-invalid': submitted && f.username.errors }" />
        <div *ngIf="submitted && f.username.errors" class="invalid-feedback">
            <div *ngIf="f.username.errors.required">Username is required</div>
        </div>
      </div>
      <div class="form-group">
        <label for="password">Password</label>
        <input type="password" formControlName="password" class="form-control" [ngClass]="{ 'is-invalid': submitted && f.password.errors }" />
        <div *ngIf="submitted && f.password.errors" class="invalid-feedback