fix(gtk): ensure GTK UIs run on main thread

[b00f]

Description

This PR introduces IdleAddSync to ensure GTK UIs run on the main thread.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.30%. Comparing base (86a8ba9) to head (de2e32c).
⚠️ Report is 298 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2081      +/-   ##
==========================================
- Coverage   75.07%   71.30%   -3.77%     
==========================================
  Files         234      293      +59     
  Lines       12156    18088    +5932     
==========================================
+ Hits         9126    12898    +3772     
- Misses       2582     4658    +2076     
- Partials      448      532      +84     

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