Fix authentication errors handling.

frontend/src/app/core/guard/authentication.guard.ts

@@ -0,0 +1,18 @@
+import { inject } from "@angular/core";
+import { CanActivateFn, Router } from "@angular/router";
+import { AuthenticationService } from "../service/authentication.service";
+import { MatSnackBar } from "@angular/material/snack-bar";
+
+export const authenticationGuard: CanActivateFn = () => {
+    const authenticationService = inject(AuthenticationService);
+    const router = inject(Router);
+    const snackBar = inject(MatSnackBar);
+
+    if (authenticationService.isAuthenticated()) {
+        return true;
+    } else {
+        router.navigate(['/login']);
+        snackBar.open('You are unauthenticated. Please, log-in first.', 'Close', { duration: 5000 });
+        return false;
+    }
+}