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/tsconfig.node.json

@@ -8,5 +8,5 @@
     "skipLibCheck": true,
     "types": ["node"]
   },
-  "include": ["vite.config.ts"]
+  "include": ["vite.config.ts", "src/lib/offer.ts"]
 }