Landing v2: icon switchers, ephemeral theme, channel link, drop browser CTA

Owner review-pass rework of the landing page:
- Rename the per-language Telegram link build var
  VITE_TELEGRAM_LINK_EN/_RU -> VITE_TELEGRAM_GAME_CHANNEL_NAME_EN/_RU
  (it carries a channel username; the landing builds https://t.me/<name> --
  the same channels the connector posts to via TELEGRAM_GAME_CHANNEL_ID_*).
- Language switcher -> a globe icon dropdown (flags + names), saved + synced
  to the app prefs.
- Theme switcher -> a sun/moon icon toggle, ephemeral (follows the system
  scheme, no auto, never persisted) -- galaxy-game style.
- Drop the "Play in browser" CTA (no standalone-web onboarding yet).

Docs: FUNCTIONAL(+ru), PLAN, deploy + ui READMEs.

ui/README.md

@@ -29,7 +29,7 @@ pnpm codegen      # regenerate src/gen from edge.proto + scrabble.fbs (dev-time)
 gateway origin for a packaged (non-proxied) build. `VITE_TELEGRAM_BOT_ID` (Stage 11)
 enables the "Link Telegram" web sign-in (the Login Widget) — inert until the site
 domain is registered with BotFather (`/setdomain`); `VITE_TELEGRAM_LINK` is the
-share-to-Telegram deep-link base (Stage 9). `VITE_TELEGRAM_LINK_EN` / `VITE_TELEGRAM_LINK_RU`
+share-to-Telegram deep-link base (Stage 9). `VITE_TELEGRAM_GAME_CHANNEL_NAME_EN` / `VITE_TELEGRAM_GAME_CHANNEL_NAME_RU`
 are the per-language "Play in Telegram" links shown on the landing page (Stage 17).
 
 The build has **two entries**: the game SPA (`index.html`, served at `/app/` and