Add common loader (#18556)

To avoid white screens on reload, building a shared skeleton.

Before

https://github.com/user-attachments/assets/42bd0667-141d-4df4-9072-4077192cc71d

After

https://github.com/user-attachments/assets/e6031a72-2e25-47e3-a873-b89aaddfbd3a

packages/twenty-front/src/modules/app/components/LazyRoute.tsx

@@ -1,9 +1,17 @@
+import { PageContainer } from '@/ui/layout/page/components/PageContainer';
 import { type ReactNode, Suspense } from 'react';
+import { PageContentSkeletonLoader } from '~/loading/components/PageContentSkeletonLoader';
 
 type LazyRouteProps = {
   children: ReactNode;
 };
 
-export const LazyRoute = ({ children }: LazyRouteProps) => (
-  <Suspense fallback={<></>}>{children}</Suspense>
+const LazyRouteFallback = () => (
+  <PageContainer>
+    <PageContentSkeletonLoader />
+  </PageContainer>
+);
+
+export const LazyRoute = ({ children }: LazyRouteProps) => (
+  <Suspense fallback={<LazyRouteFallback />}>{children}</Suspense>
 );