A native Windows MessageBox attached to a parent window disables that
window (WS_DISABLED) for its lifetime and re-enables it on dismissal.
When the parent is the main window — whose WindowClosing hook hides
instead of closes — the enable/hide sequence races and leaves the window
unable to process its close (X) button afterwards, so e.g. a rejected
login error dialog left the main window stuck open.
Route all native dialogs through src/lib/dialogs.ts, which forces
Detached: true on Windows (NULL owner, no window ever disabled) and is a
no-op on macOS/Linux (keeps the attached sheet-style presentation).
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