feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

ui/src/lib/landing.test.ts

@@ -4,17 +4,13 @@ import { telegramChannelLink } from './landing';
 describe('telegramChannelLink', () => {
   afterEach(() => vi.unstubAllEnvs());
 
-  it('builds the per-language t.me link from the channel name', () => {
-    vi.stubEnv('VITE_TELEGRAM_GAME_CHANNEL_NAME_EN', 'Scrabble_Game');
-    vi.stubEnv('VITE_TELEGRAM_GAME_CHANNEL_NAME_RU', '@Erudit_Game'); // a leading @ is tolerated
-    expect(telegramChannelLink('en')).toBe('https://t.me/Scrabble_Game');
-    expect(telegramChannelLink('ru')).toBe('https://t.me/Erudit_Game');
+  it('builds the t.me link from the channel name', () => {
+    vi.stubEnv('VITE_TELEGRAM_GAME_CHANNEL_NAME', '@Erudit_Game'); // a leading @ is tolerated
+    expect(telegramChannelLink()).toBe('https://t.me/Erudit_Game');
   });
 
-  it('returns null when the locale channel is unset or blank', () => {
-    vi.stubEnv('VITE_TELEGRAM_GAME_CHANNEL_NAME_EN', '');
-    vi.stubEnv('VITE_TELEGRAM_GAME_CHANNEL_NAME_RU', '   ');
-    expect(telegramChannelLink('en')).toBeNull();
-    expect(telegramChannelLink('ru')).toBeNull();
+  it('returns null when the channel name is unset or blank', () => {
+    vi.stubEnv('VITE_TELEGRAM_GAME_CHANNEL_NAME', '   ');
+    expect(telegramChannelLink()).toBeNull();
   });
 });