Chore: extract common immersive styling

[cstns]

Prerequisites

branched off #6621, needs to be merged after it

Description

moves immersive editor styles to a common stylesheet in preparation of the remote instance immersive mode.

Related Issue(s)

part of #6599

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 76.66%. Comparing base (fbf6154) to head (98cbdcc).

Additional details and impacted files

@@                                   Coverage Diff                                    @@
##           chore/extract-drawer-helper-functions-into-a-composable    #6622   +/-   ##
========================================================================================
  Coverage                                                    76.66%   76.66%           
========================================================================================
  Files                                                          398      398           
  Lines                                                        20103    20103           
  Branches                                                      4841     4841           
========================================================================================
  Hits                                                         15411    15411           
  Misses                                                        4692     4692           

Flag	Coverage Δ
backend	76.66% <ø> (ø)

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