Fix immersive editor not displaying due to parent container height

[cstns]

Description

makes the ff-layout-plain stretch to max available space

Related Issue(s)

introduced by #4763

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 78.79%. Comparing base (5278a37) to head (1b3bd26).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4780   +/-   ##
=======================================
  Coverage   78.79%   78.79%           
=======================================
  Files         311      311           
  Lines       14782    14782           
  Branches     3385     3385           
=======================================
  Hits        11647    11647           
  Misses       3135     3135           

Flag	Coverage Δ
backend	78.79% <ø> (ø)

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