feat(users): allow system roles to be renamed but not have permissions changed

- update `updateRole` to allow name changes for system roles while blocking permission updates
- remove edit button restriction for system roles in roles page
- disable name field only was replaced by disabling permissions checkboxes for system roles in edit modal
- update README to reflect new system role update policy

src/features/admin/components/RoleEditModal.client.js

@@ -125,7 +125,6 @@ const RoleEditModal = ({ roleId, isOpen, onClose, onSaved }) => {
                             label="Nom du rôle"
                             value={name}
                             onChange={setName}
-                            disabled={isSystem}
                             placeholder="Éditeur, Modérateur..."
                             required
                         />
@@ -162,6 +161,7 @@ const RoleEditModal = ({ roleId, isOpen, onClose, onSaved }) => {
                                             onChange={() => togglePerm(perm.key)}
                                             label={perm.name}
                                             description={perm.description}
+                                            disabled={isSystem}
                                         />
                                     ))}
                                 </div>