Fixes setMainNavBackButton race condition before a team is present after logging in

[cstns]

Description

After a consistent way of reproducing the bug was found in #4937 I was able to poke around and find the culprit.

The underlying problem is that the backToButton watcher in the mainNav component was triggered immediately when the component was mounted after login but prior of having a loaded team which caused the backButton computed prop to fail and pushing an undefined value in the ux store.

The fix is preventing the backButton to be pushed in an invalid state whilst reacting/setting the button upon team changes.

Related Issue(s)

closes #4797

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.44%. Comparing base (cf9bfbe) to head (60a0e84).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4949   +/-   ##
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  Coverage   78.44%   78.44%           
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  Files         325      325           
  Lines       15353    15353           
  Branches     3529     3529           
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  Hits        12043    12043           
  Misses       3310     3310           

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