Fix the immersive editor dashboard button

[cstns]

Description

Fixes the immersive editor's dashboard button by disabling it when the instance is not available/

Related Issue(s)

closes #3789

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[joepavitt]

code changes look good - approved subject to the CI/CD pipeline all passing okay

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.10%. Comparing base (6887f75) to head (f33e278).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3837   +/-   ##
=======================================
  Coverage   79.10%   79.10%           
=======================================
  Files         279      279           
  Lines       12524    12524           
  Branches     2782     2782           
=======================================
  Hits         9907     9907           
  Misses       2617     2617           

Flag	Coverage Δ
backend	79.10% <ø> (ø)

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