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/Landing.svelte

@@ -123,7 +123,10 @@
     </div>
   </section>
 
-  <footer class="ft">{t('about.version', { v: __APP_VERSION__ })}</footer>
+  <footer class="ft">
+    <a class="offer" href="/offer/">{t('landing.offer')}</a>
+    <span>{t('about.version', { v: __APP_VERSION__ })}</span>
+  </footer>
 </main>
 
 <style>
@@ -275,8 +278,18 @@
   }
   .ft {
     margin-top: auto;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    gap: 6px;
     text-align: center;
     color: var(--text-muted);
     font-size: 0.8rem;
   }
+  .ft .offer {
+    color: inherit;
+  }
+  .ft .offer:hover {
+    color: var(--accent);
+  }
 </style>