US-24 — Select the editions for the app alert type

## User story

As a app alert notification author, I want to select intended GU edition audiences in the app alert type.

## Editions
- [ ] United Kingdom
- [ ] United States
- [ ] Australia
- [ ] Europe
- [ ] International

## Acceptance scenarios

```gherkin
Feature: Select app alert type GU editions

Background:
Given the author is composing an app alert notification

Scenario: Select an individual GU edition (app alert type)
When the author selects e.g. "United Kingdom"
Then "United Kingdom" should be shown as selected
And the audience should include "United Kingdom"

Scenario: Select multiple GU editions (app alert type)
When the author selects e.g. "United Kingdom"
And the author selects e.g. "Australia"
Then both regions should be shown as selected
And the audience should include both regions

Scenario: Store selected audience segments
Given the author has selected one or more audience segments
When the notification is saved
Then the selected audience segments should be stored against the notification
```

**Out of scope**:

```gherkin
Scenario: Select Global
When the author selects "Global"
Then "Global" should be shown as selected
And "United Kingdom" should be selected
And "United States" should be selected
And "Australia" should be selected
And "Europe" should be selected
And "International" should be selected

Scenario: Deselect Global
Given the author has selected "Global"
When the author deselects "Global"
Then "Global" should not be selected
And the regions selected through Global should no longer be selected
```

Start by locating the app alert authoring entry point and the notification persistence path; the issue names no files or tests. Trace where edition audience selections are rendered and stored, then verify the acceptance scenarios: individual and multiple editions appear selected and persist when the notification is saved, while Global remains out of scope.

Written by the indexing model from the issue text.