docs: record the modal that hangs test_mainwindow as item 84

Cause verified by attaching gdb to the hung process rather than
inferred: showWarnings() raises QMessageBox::warning from the MainWindow
constructor, and nothing offscreen can dismiss it, so any config problem
in a test's fixture blocks the constructor forever.

Not a defect in the application. The modal is right for a person and the
code says why; the defect is that a test cannot dismiss it and the
resulting failure is a silent hang rather than an error naming the
cause.

Corrects item 81's closing note, which blamed the missing maildir key
itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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