fix(#15950): mobile favorites folder navigation with proper back button (#16118)

## What’s done
- Clicking a favorite folder on mobile now opens a clean drill-down
layer with only its contents
- Added a back button (icon + folder name) — visually 100% identical to
`NavigationDrawerBackButton`
- Sidebar no longer collapses when opening a folder
- Tree line is removed on mobile (consistent with current Twenty design)

## Intentional deviations from the mockup
1. **Back button does not replace `MultiWorkspaceDropdownButton`**  
Kept `NavigationDrawerHeader` untouched — it’s responsible for the
hamburger menu and workspace name.
   Added the back button as a separate block below.  
→ This feels cleaner architecturally and avoids breaking other places
that use `NavigationDrawerHeader`.

2. **Vertical spacing between items is slightly tighter than in the
mockup**
Used `NavigationDrawerSubItem` — the standard component for nested items
in Twenty.
Current production spacing matches this exactly, so I preferred
consistency over pixel-perfect mockup matching.

Happy to adjust either point if the team prefers to follow the mockup
1:1.

Closes #15950

---------

Co-authored-by: Baptiste Devessier <baptiste@devessier.fr>

packages/twenty-front/src/modules/navigation/components/MainNavigationDrawer.tsx

@@ -1,15 +1,23 @@
 import { useRecoilValue } from 'recoil';
 
 import { currentWorkspaceState } from '@/auth/states/currentWorkspaceState';
+import { FavoritesFolderContent } from '@/favorites/components/FavoritesFolderContent';
+import { useFavoritesByFolder } from '@/favorites/hooks/useFavoritesByFolder';
 import { MainNavigationDrawerFixedItems } from '@/navigation/components/MainNavigationDrawerFixedItems';
 import { MainNavigationDrawerScrollableItems } from '@/navigation/components/MainNavigationDrawerScrollableItems';
 import { SupportDropdown } from '@/support/components/SupportDropdown';
 import { NavigationDrawer } from '@/ui/navigation/navigation-drawer/components/NavigationDrawer';
 import { NavigationDrawerFixedContent } from '@/ui/navigation/navigation-drawer/components/NavigationDrawerFixedContent';
 import { NavigationDrawerScrollableContent } from '@/ui/navigation/navigation-drawer/components/NavigationDrawerScrollableContent';
+import { currentFavoriteFolderIdState } from '@/ui/navigation/navigation-drawer/states/currentFavoriteFolderIdState';
 
 export const MainNavigationDrawer = ({ className }: { className?: string }) => {
   const currentWorkspace = useRecoilValue(currentWorkspaceState);
+  const currentFavoriteFolderId = useRecoilValue(currentFavoriteFolderIdState);
+  const { favoritesByFolder } = useFavoritesByFolder();
+  const openedFolder = favoritesByFolder.find(
+    (f) => f.folderId === currentFavoriteFolderId,
+  );
 
   return (
     <NavigationDrawer
@@ -21,7 +29,15 @@ export const MainNavigationDrawer = ({ className }: { className?: string }) => {
       </NavigationDrawerFixedContent>
 
       <NavigationDrawerScrollableContent>
-        <MainNavigationDrawerScrollableItems />
+        {currentFavoriteFolderId && openedFolder ? (
+          <FavoritesFolderContent
+            folderName={openedFolder.folderName}
+            folderId={openedFolder.folderId}
+            favorites={openedFolder.favorites}
+          />
+        ) : (
+          <MainNavigationDrawerScrollableItems />
+        )}
       </NavigationDrawerScrollableContent>
 
       <NavigationDrawerFixedContent>