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/loading/components/UserOrMetadataLoader.tsx

@@ -13,11 +13,9 @@ const StyledContainer = styled.div`
   box-sizing: border-box;
   display: flex;
   flex-direction: row;
-  gap: 12px;
   height: 100dvh;
   min-width: ${NAVIGATION_DRAWER_CONSTRAINTS.default}px;
   overflow: hidden;
-  padding: 12px 8px 12px 8px;
   width: 100%;
 
   @media (max-width: ${MOBILE_VIEWPORT}px) {
@@ -25,6 +23,11 @@ const StyledContainer = styled.div`
   }
 `;
 
+const StyledLeftPanelWrapper = styled.div`
+  flex-shrink: 0;
+  padding: 12px 0 12px 8px;
+`;
+
 export const UserOrMetadataLoader = () => {
   const showAuthModal = useShowAuthModal();
 
@@ -36,7 +39,9 @@ export const UserOrMetadataLoader = () => {
           backdropZIndex={RootStackingContextZIndices.RootModalBackDrop}
         />
       )}
-      <LeftPanelSkeletonLoader />
+      <StyledLeftPanelWrapper>
+        <LeftPanelSkeletonLoader />
+      </StyledLeftPanelWrapper>
       <RightPanelSkeletonLoader />
     </StyledContainer>
   );