US-14 — Edit a draft notification

## User story

As a notification author, I want to edit a draft notification so that I can complete or correct it before delivery.

## Acceptance scenarios

```gherkin
Feature: Edit a draft notification

Background:
Given a notification exists with status "draft"
And the author is viewing all notifications

Scenario: Open a draft for editing
When the author selects "edit" for the draft
Then the compose page should open
And the draft's existing values should be populated

Scenario: Update a draft
Given the author has opened a draft
When the author changes the notification content
And saves the draft
Then the updated content should be stored
And the notification should remain in status "draft"

Scenario: Change draft communication channels
Given the author has opened a draft
When the author changes the selected communication channels
And saves the draft
Then the updated communication channels should be stored

Scenario: Change draft audience segments
Given the author has opened a draft
When the author changes the selected audience segments
And saves the draft
Then the updated audience segments should be stored

Scenario: Update draft audit information
When the author saves changes to the draft
Then the last modified date and time should be updated
And the current author should be recorded as the last author
```

Start by tracing the existing all-notifications view, the draft edit action, and the compose page described in the acceptance scenarios. Check how notification content, communication channels, audience segments, status, and audit information are stored; done means drafts can be reopened, updated, and saved while remaining in draft status with updated metadata.

Written by the indexing model from the issue text.