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/src/lib/landing.ts

@@ -1,16 +1,20 @@
 // Pure helpers for the public landing page (Stage 17), kept out of the Svelte component so
-// the per-language Telegram-bot link selection is unit-testable.
+// the per-language Telegram-channel link selection is unit-testable.
 
 import type { Locale } from './i18n/index.svelte';
 
 /**
- * telegramBotLink returns the t.me link for the locale's game bot, or null when it is not
- * configured. The two links are build-time vars (VITE_TELEGRAM_LINK_EN / VITE_TELEGRAM_LINK_RU)
- * because the test and prod contours run different bots (different usernames), so the link
- * cannot be hardcoded.
+ * telegramChannelLink returns the t.me link for the locale's game channel, or null when it is
+ * not configured. The channel usernames are build-time vars (VITE_TELEGRAM_GAME_CHANNEL_NAME_EN
+ * / VITE_TELEGRAM_GAME_CHANNEL_NAME_RU) because the test and prod contours run different
+ * channels; they are the same channels the connector posts to via TELEGRAM_GAME_CHANNEL_ID_*
+ * (the id to post, the name to link). A leading "@" is tolerated.
  */
-export function telegramBotLink(locale: Locale): string | null {
-  const raw = locale === 'ru' ? import.meta.env.VITE_TELEGRAM_LINK_RU : import.meta.env.VITE_TELEGRAM_LINK_EN;
-  const link = (raw as string | undefined)?.trim();
-  return link ? link : null;
+export function telegramChannelLink(locale: Locale): string | null {
+  const raw =
+    locale === 'ru'
+      ? import.meta.env.VITE_TELEGRAM_GAME_CHANNEL_NAME_RU
+      : import.meta.env.VITE_TELEGRAM_GAME_CHANNEL_NAME_EN;
+  const name = (raw as string | undefined)?.trim().replace(/^@/, '');
+  return name ? `https://t.me/${name}` : null;
 }