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/codec.ts

@@ -294,17 +294,13 @@ function decodeChatMsg(m: fb.ChatMessage): ChatMessage {
 }
 
 // sessionFromTable projects a FlatBuffers Session table (a root or a nested one) to
-// the Session model, including the supported-languages set the UI gates variants by.
+// the Session model.
 function sessionFromTable(t: fb.Session): Session {
-  const supportedLanguages: string[] = [];
-  for (let i = 0; i < t.supportedLanguagesLength(); i++) supportedLanguages.push(s(t.supportedLanguages(i)));
   return {
     token: s(t.token()),
     userId: s(t.userId()),
     isGuest: t.isGuest(),
     displayName: s(t.displayName()),
-    supportedLanguages,
-    serviceLanguage: s(t.serviceLanguage()),
   };
 }
 
@@ -326,10 +322,19 @@ export function decodeProfile(buf: Uint8Array): Profile {
     blockFriendRequests: p.blockFriendRequests(),
     isGuest: p.isGuest(),
     notificationsInAppOnly: p.notificationsInAppOnly(),
+    variantPreferences: decodeVariantPreferences(p),
     banner: decodeBanner(p),
   };
 }
 
+// decodeVariantPreferences reads the Profile.variant_preferences vector (the variants
+// the player enabled in Settings).
+function decodeVariantPreferences(p: fb.Profile): Variant[] {
+  const out: Variant[] = [];
+  for (let i = 0; i < p.variantPreferencesLength(); i++) out.push(s(p.variantPreferences(i)) as Variant);
+  return out;
+}
+
 // decodeBanner projects the optional advertising-banner block of a Profile, or
 // undefined when the viewer is not eligible (the field is absent).
 function decodeBanner(p: fb.Profile): Banner | undefined {
@@ -642,6 +647,10 @@ export function encodeUpdateProfile(p: ProfileUpdate): Uint8Array {
   const tz = b.createString(p.timeZone);
   const as = b.createString(p.awayStart);
   const ae = b.createString(p.awayEnd);
+  const prefs = fb.UpdateProfileRequest.createVariantPreferencesVector(
+    b,
+    p.variantPreferences.map((v) => b.createString(v)),
+  );
   fb.UpdateProfileRequest.startUpdateProfileRequest(b);
   fb.UpdateProfileRequest.addDisplayName(b, name);
   fb.UpdateProfileRequest.addPreferredLanguage(b, lang);
@@ -651,6 +660,7 @@ export function encodeUpdateProfile(p: ProfileUpdate): Uint8Array {
   fb.UpdateProfileRequest.addBlockChat(b, p.blockChat);
   fb.UpdateProfileRequest.addBlockFriendRequests(b, p.blockFriendRequests);
   fb.UpdateProfileRequest.addNotificationsInAppOnly(b, p.notificationsInAppOnly);
+  fb.UpdateProfileRequest.addVariantPreferences(b, prefs);
   return finish(b, fb.UpdateProfileRequest.endUpdateProfileRequest(b));
 }