Allow initial Admin to modify default stack

[hardillb]

fixes #6059

Description

This allows the inital admin to modify the default Stack as part of the install wizard, it also helps to make them aware Stacks exist

Related Issue(s)

#6059

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
• Link to Changelog Entry PR, or note why one is not needed.

Labels

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.22%. Comparing base (375f5e6) to head (bb0b33a).
⚠️ Report is 159 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6326   +/-   ##
=======================================
  Coverage   76.22%   76.22%           
=======================================
  Files         393      393           
  Lines       19777    19789   +12     
  Branches     4741     4744    +3     
=======================================
+ Hits        15075    15085   +10     
- Misses       4702     4704    +2     

Flag	Coverage Δ
backend	76.22% <100.00%> (+<0.01%)	⬆️

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