Add a default-OFF 'project_quick_create_buttons' setting that gates the
per-project '+' quick-create buttons on sidebar project chips. The chip
render site only calls _attachProjectQuickCreateButton when the new
window._projectQuickCreate runtime flag is enabled, so the buttons do
not appear unless the user opts in via Settings.
Mirrors the large_text_paste_as_attachment opt-in plumbing (defaults,
allowlist, settings collect/restore/save, window hydration at both sites,
checkbox row, i18n across all locales).
