feat(ui): serve the public offer at /offer/ with a landing footer link

The offer is the legal document a purchase accepts, needed before real-money
intake goes live. Convert the source PDF to an editable ui/legal/offer_ru.md and
render it to a standalone static dist/offer/index.html at build (a vite
emit-offer plugin using marked); the landing container serves it at /offer/,
with a bare /offer redirecting in. Add a small centered "Публичная оферта"
footer link on the landing (ru/en) and a CI probe asserting /offer/ serves the
page rather than silently falling through to the landing shell.

ui/src/lib/i18n/ru.ts

@@ -275,6 +275,7 @@ export const ru: Record<MessageKey, string> = {
   'landing.captionTelegram': 'Telegram',
   'landing.captionVK': 'VK',
   'landing.captionWeb': 'Веб-версия',
+  'landing.offer': 'Публичная оферта',
 
   'install.title': 'Установить приложение',
   'install.subtitle': 'Поместите иконку приложения на рабочий стол (домашний экран), чтобы открывать игру одним нажатием.',