fix: use DS classes for login submit button

Updates the login form submit button to use DS button classes while preserving the existing form markup and submission behavior.

Author: tischsoic

src/bundle/Core/Resources/views/Security/login.html.twig

@@ -29,7 +29,7 @@
                         is redirected to on success (more details below)
                         <input type="hidden" name="_target_path" value="/account" />
                     #}
-                    <button type="submit" class="btn ibexa-btn ibexa-btn--primary">{{ 'Login'|trans }}</button>
+                    <button type="submit" class="ids-btn ids-btn--primary ids-btn--medium">{{ 'Login'|trans }}</button>
                 </fieldset>
             {% endblock %}
         </form>