OZG-7773 small improvements

...ent/libs/postfach-shared/src/lib/postfach.service.spec.ts

alfa-client/libs/postfach-shared/src/lib/postfach.service.ts

alfa-client/libs/postfach-shared/test/postfach.ts

...iner/postfach-mail-list/postfach-mail-list.component.html

...d-button/mail-unread-button-link-container.component.html

...utton/mail-unread-button-link-container.component.spec.ts

...ead-button/mail-unread-button-link-container.component.ts

...ach-page-mail-list/postfach-page-mail-list.component.html

...page-container/postfach-page/postfach-page.component.html