Add missing page and of i18n keys for submit form image alt text

The submit_form/show.html.erb uses t('page') and t('of') to build
accessible alt text for document page images (e.g. "Page 1 of Contract").
These keys were added as part of a11y work but never added to the locale file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/locales/i18n.yml

@@ -361,6 +361,8 @@ en: &en
   sign_out: Sign out
   user_menu: User menu
   page_number: 'Page %{number}'
+  page: Page
+  of: of
   powered_by: Powered by
   count_documents_signed_with_html: '<b>%{count}</b> documents signed with'
   storage: Storage