* fix(ui): use Gtk response signal for first-run dialog
Overriding do_response and calling Gtk.Dialog.do_response() breaks on
modern PyGObject with "Class GtkDialog doesn't implement response".
Connect to the response signal instead so dialog.run() still works.
Fixes#566
* test(ui): cover first-run dialog response handler
Exercise connect("response") wiring and _on_response mapping so
codecov/patch is green for the #566 Gtk.Dialog.do_response fix.
* test(ui): isolate first-run dialog response coverage
Load first_run_dialog under a unique module name with a real Gtk.Dialog
stand-in so _on_response and connect("response") run under full-suite
import pollution without clobbering other tests' gi mocks.
* test(ui): cover _on_response via SimpleNamespace
Call FirstRunDialog._on_response on a stand-in object instead of
constructing Gtk.Dialog; enough for codecov/patch on the handler body.
* test(ui): map first-run responses via SimpleNamespace
Exercise FirstRunDialog._on_response for YES/NO/LATER/unknown without
constructing Gtk.Dialog.
* test(ui): collapse first-run response cases into one table
One loop covers YES/NO/LATER/unknown instead of four copy-pasted tests.
* test(ui): restore isolated first-run dialog coverage load
SimpleNamespace calls against vocalinux.ui.first_run_dialog.FirstRunDialog
fail under the full suite when earlier tests leave the class as a MagicMock.
Reload the source under a unique module name with a real Gtk.Dialog stand-in
(as in 680c5ed) so _on_response and connect("response") stay green on CI.
