feat: Display assigned user groups in Admin Panel

Description:
This PR adds the ability to view a user’s assigned groups in the Admin Panel when editing a user.

Backend Changes:
    Added a new endpoint:
    GET /api/v1/users/{user_id}/groups

        Returns the list of groups assigned to a specific user.
        Requires admin privileges.

Frontend Changes:
    Implemented getUserGroupsById API function to call the new backend endpoint, in lib/apis/users.

    Updated EditUserModal.svelte to:
        Load user groups asynchronously when the modal is opened.
        Display the groups inline in the form before the Save button.
        Show a loading state while fetching, and a “No groups assigned” message if none exist.

Result:
Admins can now see which groups a user belongs to directly from the edit user modal,
improving visibility and reducing the need to navigate away for group membership checks.

backend/open_webui/routers/users.py

@@ -501,3 +501,13 @@ async def delete_user_by_id(user_id: str, user=Depends(get_admin_user)):
         status_code=status.HTTP_403_FORBIDDEN,
         detail=ERROR_MESSAGES.ACTION_PROHIBITED,
     )
+
+
+############################
+# GetUserGroupsById
+############################
+
+
+@router.get("/{user_id}/groups")
+async def get_user_groups_by_id(user_id: str, user=Depends(get_admin_user)):
+    return Groups.get_groups_by_member_id(user_id)