fix: Reimplement responsive composer layout

[Septicity]

Description

In the replacement of the Slate-based message editor with ProseMirror, there was a regression regarding the composer layout responding to the size of the entered text.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

I determined the exact range within Editor.tsx that previously contained the logic that dealt with the responsive layout, but it was quite dense and interspersed, so I had GitHub Copilot figure out the ideal way to reimplement the logic and tests from the previous commits into the new system. As far as I can tell, it creates multiple references that measure different aspects and actions on the message editor on each update to determine when and how to resize it.

[eleboucher]

it should be a regular body