US-16 — Protect dispatched notifications from editing

## User story

As a notification author, I want dispatched notifications to be protected from modification so that the historical record remains accurate.

## Acceptance scenarios

```gherkin
Feature: Protect dispatched notifications

Scenario: Do not display an edit action for a dispatched notification
Given a notification has status "dispatched"
When the author views the notification list
Then an edit action should not be available for that notification

Scenario: Attempt to access the edit URL directly
Given a notification has status "dispatched"
When the author attempts to access its edit page directly
Then the notification should not be editable
And the notification should be displayed as read-only
```

Start with the notification list and edit-page flows described in the acceptance scenarios, then locate the status handling for dispatched notifications. Run the relevant existing test suite or add focused coverage for both scenarios. Done means dispatched notifications have no edit action, direct edit access is read-only, and other statuses retain their existing behavior.

Written by the indexing model from the issue text.