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.

.gitea/workflows/ci.yaml

@@ -497,6 +497,22 @@ jobs:
           docker logs --tail 50 scrabble-backend || true
           exit 1
 
+      - name: Probe the /offer/ public offer page is served
+        run: |
+          set -u
+          # /offer/ is a static page baked into the landing image (rendered from
+          # ui/legal/offer_ru.md). If the landing Caddyfile stops routing it, the request
+          # silently falls through to the landing shell (also 200) — so assert offer-specific
+          # content, never just the status.
+          out="$(docker run --rm --network edge alpine:3.20 wget -q -O - http://scrabble/offer/ 2>&1 || true)"
+          if echo "$out" | grep -q "290210610742"; then
+            echo "ok: /offer/ serves the public offer page"
+          else
+            echo "FAIL: /offer/ did not serve the offer page (fell through to the landing shell?)"
+            docker logs --tail 50 scrabble-landing || true
+            exit 1
+          fi
+
       - name: Probe the /dict edge route reaches the gateway
         run: |
           set -u