Move the last few user-visible hardcoded strings into vue-i18n:
- AppHeader: "Toggle Menu" / "User Menu" aria-labels (screen readers
read these in English regardless of locale)
- CustomPageView: "Page not found" error body
- PaymentProviderDialog: display-name example placeholder hardcoded in
Chinese for all locales
Adds common.toggleMenu / common.userMenu / common.pageNotFound and
admin.settings.payment.customMethodDisplayNamePlaceholder to both zh
and en locales, and registers the new placeholder key in the
PaymentProviderDialog spec's i18n mock.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>