Dashboard: Use persistent notification pattern for command results instead of modal dialog

[davidfowl]

Summary

Currently, when a resource command returns result data (text or JSON), the Dashboard opens a TextVisualizerDialog modal immediately after the command completes (#15622). This can be disruptive — with a long-running command, a modal dialog may open unexpectedly while the user is doing something else in the dashboard, and they might not care to look at the result.

Proposal

Replace the modal dialog with a persistent notification pattern, similar to what Azure Portal and other web applications use:

1. Add a notification icon in the dashboard header that shows recent notifications
2. When a command completes with result data, add a notification entry instead of opening a modal
3. The user can click on a notification to view the result (opening the TextVisualizerDialog on demand)
4. Notifications should show whether the command succeeded or failed
5. Unread notifications should be visually indicated (badge count, etc.)

This gives users control over when they view command results rather than interrupting their workflow.

Context

From PR review discussion in #15622 — @JamesNK suggested this approach:

I think what we should have long term is a notification icon in the header that opens and displays recent notifications. From here you could choose to look at the result from a command. Basically, copy what Azure Portal (and many other websites) do with notifications for async tasks.

Additional considerations

• The notification list could also be useful for other async operations beyond commands
• Consider how notifications interact with the existing toast system
• Notifications should persist across page navigation within the dashboard session