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.

deploy/.env.example

@@ -25,8 +25,8 @@ GM_BASICAUTH_HASH=                     # required; `caddy hash-password` bcrypt
 # --- UI build args (baked into the gateway image) ---------------------------
 VITE_TELEGRAM_BOT_ID=
 VITE_TELEGRAM_LINK=
-VITE_TELEGRAM_LINK_EN=                 # landing "Play in Telegram" link, English bot
-VITE_TELEGRAM_LINK_RU=                 # landing "Play in Telegram" link, Russian bot
+VITE_TELEGRAM_GAME_CHANNEL_NAME_EN=                 # landing "Play in Telegram" link, English bot
+VITE_TELEGRAM_GAME_CHANNEL_NAME_RU=                 # landing "Play in Telegram" link, Russian bot
 VITE_GATEWAY_URL=
 
 # --- Gateway ----------------------------------------------------------------