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/e2e/landing.spec.ts

@@ -58,3 +58,14 @@ test('the landing shows a web-version entry linking /app/, with a caption', asyn
   expect(await web.getAttribute('href')).toContain('/app/');
   await expect(page.getByText('Веб-версия')).toBeVisible();
 });
+
+// The footer carries a public-offer link to the static /offer/ page (rendered from
+// ui/legal/offer_ru.md at build; the legal document a purchase accepts).
+test('the landing footer links to the public offer at /offer/', async ({ page }) => {
+  await page.goto('/landing.html');
+  await expect(page.getByText(/Играй в «Эрудита»/)).toBeVisible(); // Russian by default
+
+  const offer = page.getByRole('link', { name: 'Публичная оферта' });
+  await expect(offer).toBeVisible();
+  expect(await offer.getAttribute('href')).toBe('/offer/');
+});