Fix UI bug in REST API playground (#10848)

When you clicked on the left-side menu to navigate, the top bar would
disappear. This PR fixes that and the top bar always remain visible.

packages/twenty-front/src/modules/ui/layout/fullscreen/components/FullScreenContainer.tsx

@@ -1,4 +1,7 @@
-import { PageHeader } from '@/ui/layout/page/components/PageHeader';
+import {
+  PAGE_BAR_MIN_HEIGHT,
+  PageHeader,
+} from '@/ui/layout/page/components/PageHeader';
 import {
   Breadcrumb,
   BreadcrumbProps,
@@ -23,7 +26,9 @@ const StyledFullScreen = styled.div`
 
 const StyledMainContainer = styled.div`
   border-top: 1px solid ${({ theme }) => theme.border.color.medium};
-  height: 100%;
+  height: calc(
+    100% - ${PAGE_BAR_MIN_HEIGHT}px - ${({ theme }) => theme.spacing(2 * 2)}
+  );
   width: 100%;
 `;