US-27 — See an indicative app alert rendered preview

## User story

As an app alert notification author, I want to see an indicative preview of what the app alert will look like.

## Acceptance scenarios

```gherkin
Feature: Preview an app alert notification

Background:
Given the author is composing content for the app alert

Scenario: Alert type and headline content preview are shown
When an author has imported valid content for an app alert
And has selected the alert type e.g. "Breaking News"
And has selected the GU Edition, e.g. "United States"
Then the send info, alert type and edition(s) selected will be shown in the message preview along with an info label
And the user will be able to see what the app alert will look like on iOS and Android

```

Nb;
- Audience segments s/b Editions
- Info Label should say "App alert formats might differ across platforms and devices"

Start at the app-alert composition flow and trace how imported content, alert type, and Editions are represented. Locate the existing message-preview entry point, if present, and determine how iOS and Android previews are rendered. Done means the preview shows the send information, selected alert type and Editions, includes the specified info label, and represents both platforms.

Written by the indexing model from the issue text.