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/package.json

@@ -28,6 +28,7 @@
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@types/node": "^22.10.0",
     "core-js-bundle": "^3.49.0",
+    "marked": "^18.0.5",
     "svelte": "^5.15.0",
     "svelte-check": "^4.1.0",
     "typescript": "^5.7.0",