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.

deploy/landing/Caddyfile

@@ -18,10 +18,25 @@
 	@shell not path /assets/*
 	header @shell Cache-Control "no-cache"
 
+	# The static public offer page, rendered from ui/legal/offer_ru.md at build
+	# time into dist/offer/index.html (vite emit-offer plugin). Served with its own
+	# index so /offer/ resolves to /srv/offer/index.html rather than falling to the
+	# landing shell below (whose index is landing.html). A bare /offer redirects in.
+	handle /offer {
+		redir * /offer/ permanent
+	}
+	handle /offer/* {
+		file_server {
+			index index.html
+		}
+	}
+
 	# An unknown path falls back to the landing shell (the gateway's old "/"
 	# behaviour); "/" itself resolves through the index below.
-	try_files {path} /landing.html
-	file_server {
-		index landing.html
+	handle {
+		try_files {path} /landing.html
+		file_server {
+			index landing.html
+		}
 	}
 }