UI5 library: Handle `luigi.updateUserSettings` custom message from MFE in user settings dialog

[JohannesDoberer]

Description

When a custom MFE is rendered inside the user settings dialog (via viewUrl), it can send a custom message with id: 'luigi.updateUserSettings' to update settings data without requiring the user to click "Save". This is not handled in the UI5 library.

How it works in Luigi Core (old)

In core/src/UserSettingsDialog.svelte and core/src/App.svelte, a custom message listener is registered for the key luigi.updateUserSettings. When received, the internal user settings state is updated with the message data.

Current behavior in UI5 library

Core-modular (container) already dispatches custom-message events when an MFE calls LuigiClient.sendCustomMessage(...). However, the UI5 library's UserSettingsDialog.svelte does not listen for this event on the luigi-container it mounts for viewUrl groups.

Expected behavior

The library should:

1. Listen on the custom-message event from luigi-container instances mounted in user settings MFE groups
2. Check if event.detail.id === 'luigi.updateUserSettings'
3. Merge event.detail.data into the internal storedUserSettings state

Where to implement

src/components/userSettings/UserSettingsDialog.svelte — in the mountMfeContainers() function where luigi-container elements are created via renderMicroFrontendContainer.

Original test reference

luigirepo/test/e2e-test-application/cypress/e2e/tests/1-angular/user_settings_dialog.cy.js — test: "Ctx update after storing user settings data using custom messages"