[az][gui] Failed to mount ...: The following error occurred: instance ... is not available

**Describe the bug**
The button to 'Add mount' is not disabled when the instance is in a disabled zone.

**To Reproduce**
1. Disable the zone the instance is in.
2. Go to the Details section of the instance
3. The 'Add mount' button is still enabled (contrarily to the 'Configure' ones)
4. If you click on it, you get the 'Failed to mount' error.

**Expected behavior**
The 'Add mount' button should not be enabled when the instance is unreachable.

**Logs**
Please provide logs from the daemon, see [Access logs](https://canonical.com/multipass/docs/stable/how-to-guides/troubleshoot/access-logs/) on where to find them on your platform.

**Additional info**
- OS: Win 11 Pro
- CPU architecture or model: x64
- `multipass version`
multipass 1.17.0-dev.ci6331+ga58dcf2e.win
multipassd 1.17.0-dev.ci6331+ga58dcf2e.win

**Additional context**

Reproduce this in the GUI by disabling the instance's zone, opening its Details section, and comparing the enabled state of Add mount with the Configure buttons. Trace the GUI behavior for the instance actions and make Add mount unavailable when the instance is unreachable; done means clicking it no longer produces the Failed to mount error.

Written by the indexing model from the issue text.