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/screens/Friends.svelte

@@ -4,8 +4,7 @@
   import { connection } from '../lib/connection.svelte';
   import { gateway } from '../lib/gateway';
   import { GatewayError } from '../lib/client';
-  import { localeFrom, t } from '../lib/i18n/index.svelte';
-  import { translate } from '../lib/i18n/catalog';
+  import { t } from '../lib/i18n/index.svelte';
   import { friendCodeParam, shareLink } from '../lib/deeplink';
   import { shareTelegramLink } from '../lib/telegram';
   import type { AccountRef, FriendCode, RobotBlockEntry } from '../lib/model';
@@ -95,10 +94,8 @@
   // shareInvite shares the friend-code deep link: inside Telegram via the native
   // "share to chat" picker; on the web via the system share sheet; failing both, it
   // copies the link to the clipboard.
-  async function shareInvite(url: string, lang: string) {
-    // The caption is in the bot's language (Эрудит for ru, Scrabble for en), not the
-    // interface language — the recipient lands in that bot.
-    const text = translate(localeFrom(lang), 'friends.inviteText');
+  async function shareInvite(url: string) {
+    const text = t('friends.inviteText');
     if (shareTelegramLink(url, text)) return;
     if (typeof navigator !== 'undefined' && navigator.share) {
       try {
@@ -134,8 +131,7 @@
         <button class="btn" onclick={redeem} disabled={!connection.online}>{t('friends.redeem')}</button>
       </div>
       {#if code}
-        {@const lang = app.session?.serviceLanguage || app.locale}
-        {@const tg = shareLink(friendCodeParam(code.code), lang)}
+        {@const tg = shareLink(friendCodeParam(code.code))}
         <div class="code" data-testid="friend-code">
           <div class="coderow">
             <button class="codeval" onclick={copyCode}>{code.code}</button>
@@ -145,7 +141,7 @@
             {t('friends.codeHint')} · {t('friends.codeExpires', { time: codeTime(code.expiresAtUnix) })}
           </span>
           {#if tg}
-            <button type="button" class="link tgshare" onclick={() => shareInvite(tg, lang)}>{t('friends.shareTelegram')}</button>
+            <button type="button" class="link tgshare" onclick={() => shareInvite(tg)}>{t('friends.shareTelegram')}</button>
           {/if}
         </div>
       {:else}