Add system-ui as a backup font for heebo (to match internal font)

[cstns]

Description

Related Issue(s)

closes #4944

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 78.59%. Comparing base (3a30552) to head (f56aeea).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4946   +/-   ##
=======================================
  Coverage   78.59%   78.59%           
=======================================
  Files         325      325           
  Lines       15296    15296           
  Branches     3512     3512           
=======================================
  Hits        12022    12022           
  Misses       3274     3274           

Flag	Coverage Δ
backend	78.59% <ø> (ø)

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[Steve-Mcl]

Approved, awaiting tests passing