Notifications to instances without access lead to 404 pages

### Current Behavior

Users with dashboard roles are receiving crash notifications from instances they cannot access and are routed to a 404 page when they click on the notification. If the instance in question doesn't even have a dashboard installed so it doesn't appear in the list of available instances for the user in question, which makes it even more confusing.

### Expected Behavior

Users should either not receive these notifications or not get routed to the instance when they click the notification.

### Steps To Reproduce

1. Create a team with two instances, one of which with NR dashboard installed
2. Assign a user with the role of dashboard to the team
3. Force both instances to crash so the users get notified
4. check the notifications with the user with dashboard role and click the notifications, you should be redirected to a 404 page

### Environment

- FlowFuse version: 2.10.0
- Node.js version: N/A
- npm version: N/A
- Platform/OS: N/A
- Browser: N/A

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start by reproducing the notification flow from the listed steps, then trace how crash notifications build their destination for dashboard-role users and how instance access is checked. Done means an inaccessible instance either produces no notification or cannot send the user to a 404 page.

Written by the indexing model from the issue text.