The first-run dialog was crashing on Fedora/Wayland when clicking any
button with the error: gtk_widget_get_scale_factor: assertion
'GTK_IS_WIDGET (widget)' failed.
Root cause: Direct destroy() calls from button click handlers caused
race conditions where GTK tried to access widget properties after the
widget was marked for destruction but before event processing completed.
Fix: Use GTK's proper dialog response signal pattern instead of direct
destroy() calls. Buttons are now added with response IDs, and the
response signal handler sets the result. This lets GTK manage the
dialog lifecycle properly and prevents the race condition.
Fixes#267
