feat(landing): Russian default + SEO head, icons and OG card

The landing now always opens in Russian (saved 🌐 choice still wins) —
browser-language detection made the indexed content nondeterministic
(Googlebot renders with en-US). landing.html gains the static Russian
SEO head: title/description, canonical pinned to the production origin,
Open Graph card (Telegram/VK link previews), twitter:card, JSON-LD,
theme-color and the favicon set; the SPA shell turns noindex and its
tab title becomes «Эрудит (Скрэббл)». New assets/icons generator
(same tile design as the VK loader) produces favicon.svg/ico,
apple-touch-icon.png and og-image.png into ui/public/, plus robots.txt.

docs/FUNCTIONAL.md

@@ -23,7 +23,13 @@ variant's alphabet, remembers answers within the session and rate-limits repeats
 A public **landing page** at the site root introduces the game, switches language and
 theme, and links to the Telegram game channel and the VK Mini App; the game itself runs at
 `/app/` (web), `/telegram/` (the Telegram Mini App) and `/vk/` (the VK Mini App). The landing's theme is ephemeral
-(it follows the system scheme, not the saved preference); its language choice is saved.
+(it follows the system scheme, not the saved preference); its language choice is saved. The
+landing always opens in **Russian** — the browser language is never auto-detected (crawlers
+render with arbitrary languages, so detection would make the indexed content
+nondeterministic); a saved 🌐 choice still wins. The page carries a static Russian SEO head
+(title/description, the Open Graph card Telegram/VK link previews use, a canonical link
+pinned to the production origin, JSON-LD, the favicon set and `robots.txt`), while the SPA
+shell is `noindex` — the landing is the only indexable page.
 
 ### First-run onboarding
 The first time a player opens the app it walks them through the interface with a light