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/assets/styles.scss

@@ -269,6 +269,16 @@ main,
   overflow: hidden;
 }
 
+/* Hauptinhalt in der App-Shell: nur verbleibenden Platz einnehmen, nicht 100vh —
+   sonst wächst main mit dem Inhalt über den Viewport hinaus und .contentscroll scrollt nicht. */
+.app-shell > .app-shell__content.app-content,
+.app-shell > .app-shell__content.contenthidden {
+  flex: 1 1 0%;
+  min-height: 0;
+  height: auto;
+  max-height: none;
+}
+
 .contentscroll {
   width: 100%;
   height: 100%;