feat(admin): add user role dialog

Admins change a user's role and see the change log. The last admin
gets a clear error when deleting their own account.

src/components/DeleteAccountCard.jsx

@@ -48,6 +48,8 @@ export default function DeleteAccountCard({email}) {
             }
             if (response.status === 403) {
                 toast.error(t('SETTINGS.DELETE_ACCOUNT.WRONG_PASSWORD'));
+            } else if (response.status === 409) {
+                toast.error(t('SETTINGS.DELETE_ACCOUNT.LAST_ADMIN'));
             } else if (response.status === 429) {
                 const minutes = Math.ceil(Number(response.headers.get('Retry-After') ?? 900) / 60);
                 toast.error(t('SETTINGS.DELETE_ACCOUNT.TOO_MANY', {minutes}));