fix(ui): update styles and localization in UsersView and AppShell

- Added styles to ensure the app shell content occupies only the remaining space, preventing overflow issues.
- Updated the header in UsersView to reflect accurate localization for user administration.

frontend/src/views/admin/UsersView.vue

@@ -2,7 +2,7 @@
   <div class="admin-users">
     <section class="admin-users__hero surface-card">
       <span class="admin-users__eyebrow">Administration</span>
-      <h1>{{ $t('navigation.m-administration.useradministration') }}</h1>
+      <h1>{{ $t('navigation.m-administration.users') }}</h1>
       <p>Benutzer suchen, Kerndaten anpassen und Sperrstatus direkt im System pflegen.</p>
     </section>