[7521] Sync FlowFuse theme preference into the embedded Node-RED editor

[n-lark]

Description

See #7380 (comment)

Depends on Node-RED work: #7522 (PR here: node-red/node-red#5819)

⚠️ This is not fully tested on a remote device - its the same path as hosted instances I just couldn't figure out how to get a device to point to my local node-RED like I could with hosted instances. If anyone has any tips lmk.

Related Issue(s)

Resolves #7521
Resolves #7380

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 76.84%. Comparing base (d2b69d9) to head (f3889d1).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7523   +/-   ##
=======================================
  Coverage   76.84%   76.84%           
=======================================
  Files         410      410           
  Lines       20922    20922           
  Branches     5093     5093           
=======================================
  Hits        16078    16078           
  Misses       4844     4844           

Flag	Coverage Δ
backend	76.84% <ø> (ø)

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