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.

docs/FUNCTIONAL.md

@@ -22,8 +22,9 @@ Settings also pick the board's bonus-label style (beginner / classic / none). A
 costs nothing when the rack has no legal move. The word-check accepts only the
 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 into the web app or the matching Telegram bot; the game itself runs at
-`/app/` (web) and `/telegram/` (the Telegram Mini App).
+theme, and links to the matching per-language Telegram channel; the game itself runs at
+`/app/` (web) and `/telegram/` (the Telegram Mini App). The landing's theme is ephemeral
+(it follows the system scheme, not the saved preference); its language choice is saved.
 
 ### Identity & sessions *(Stage 1 / 6 / 9 / 15)*
 A player arrives from a platform (Telegram first), via email login, or as an