Convert observables to signals.

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

@@ -8,7 +8,7 @@
             <header>
                 <h1>{{ publication.title }}</h1>
                 <h2>{{ publication.description }}</h2>
-                @if (isAuthorAndUserEquals) {
+                @if (isAuthorAndUserEquals()) {
                     <a [routerLink]="['edit']"
                         class="button action"
                         matTooltip="Click to edit the publication"