US-15 — Edit a scheduled notification

## User story

As a notification author, I want to edit a scheduled notification before it is dispatched so that I can correct its content, routing or delivery time.

## Acceptance scenarios

```gherkin
Feature: Edit a scheduled notification

Background:
Given a notification exists with status "scheduled"

Scenario: Open a scheduled notification for editing
When the author selects "edit"
Then the compose page should display its current content
And its audience segments should be displayed
And its communication channels should be displayed
And its scheduled delivery date and time should be displayed

Scenario: Update scheduled notification content
Given the scheduled notification has not been dispatched
When the author changes its content
And saves the changes
Then the updated content should be stored
And the notification should remain in status "scheduled"

Scenario: Update the scheduled delivery time
Given the scheduled notification has not been dispatched
When the author selects a different valid future delivery time
And saves the changes
Then the new scheduled time should be stored
And the notification should remain in status "scheduled"

Scenario: Update scheduled communication channels
Given the scheduled notification has not been dispatched
When the author changes its communication channels
And saves the changes
Then the updated communication channels should be stored

Scenario: Attempt to edit after dispatch has begun
Given the notification is already being dispatched
When the author attempts to save changes
Then the changes should not be applied
And the author should be informed that the notification can no longer be edited
```

Start by locating the compose page and the code that saves scheduled notifications, then trace how dispatch status is checked. Done means authors can edit content, audience segments, channels, and future delivery time while the notification remains scheduled, and saves are rejected with an explanation once dispatch has begun.

Written by the indexing model from the issue text.