refactor(ui): update layout styles for AppContent and AccountView components

- Changed layout from grid to flex in AccountView for improved responsiveness and alignment.
- Adjusted flex properties in AppContent to ensure proper scrolling behavior and height management.
- Enhanced styling for account settings panel to support better content overflow handling and visual consistency.

frontend/src/components/AppContent.vue

@@ -45,8 +45,9 @@
   flex-direction: column;
 }
 
+/* flex-basis 0: Höhe richtet sich nach verbleibendem Platz, nicht nach Inhalt (sonst kein Scroll) */
 .app-content__inner > :last-child {
-  flex: 1 1 auto;
+  flex: 1 1 0%;
   min-height: 0;
 }