fix: add missing folders field to SharingPermissions so the permission persists (#27296)

`sharing.folders` is present in `DEFAULT_USER_PERMISSIONS` but absent from the `SharingPermissions` response/update schema, so the admin default and group permission API silently drops it on every round-trip and the setting is never saved. Add `folders: bool = False`, matching the config default (`USER_PERMISSIONS_FOLDERS_ALLOW_SHARING`), restoring parity with `DEFAULT_USER_PERMISSIONS`.

Fixes #27120.

backend/open_webui/routers/users.py

@@ -200,6 +200,7 @@ class SharingPermissions(BaseModel):
     public_skills: bool = False
     notes: bool = False
     public_notes: bool = True
+    folders: bool = False
     public_chats: bool = False
     public_calendars: bool = False