feat(App): improve layout responsiveness and overflow handling

- Added min-width and max-width properties to App.vue for better layout control.
- Enhanced main-content and content classes with overflow-x hidden and box-sizing border-box to improve responsiveness and prevent overflow issues.

frontend/src/App.vue

@@ -666,6 +666,8 @@ export default {
   flex: 1;
   overflow: hidden;
   min-height: 0;
+  min-width: 0;
+  max-width: 100%;
 }
 
 /* Sidebar */
@@ -881,8 +883,11 @@ export default {
 .main-content {
   flex: 1;
   overflow-y: auto;
+  overflow-x: hidden;
   background: var(--bg-canvas);
   min-height: 0;
+  min-width: 0;
+  max-width: 100%;
   padding-bottom: 32px; /* Platz für Statusleiste (24px + 8px padding) */
 }
 
@@ -918,6 +923,9 @@ export default {
 .content {
   padding: 1.5rem;
   min-height: 100%;
+  min-width: 0;
+  max-width: 100%;
+  box-sizing: border-box;
 }
 
 .content.content-full-height {