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.test.ts

@@ -26,6 +26,7 @@ import {
   encodeStateRequest,
   encodeSubmitPlay,
   encodeTarget,
+  encodeUpdateProfile,
 } from './codec';
 
 describe('codec', () => {
@@ -188,21 +189,17 @@ describe('codec', () => {
     const token = b.createString('tok');
     const uid = b.createString('u1');
     const name = b.createString('Me');
-    const langs = fb.Session.createSupportedLanguagesVector(b, [b.createString('en'), b.createString('ru')]);
     fb.Session.startSession(b);
     fb.Session.addToken(b, token);
     fb.Session.addUserId(b, uid);
     fb.Session.addIsGuest(b, true);
     fb.Session.addDisplayName(b, name);
-    fb.Session.addSupportedLanguages(b, langs);
     b.finish(fb.Session.endSession(b));
     expect(decodeSession(b.asUint8Array())).toEqual({
       token: 'tok',
       userId: 'u1',
       isGuest: true,
       displayName: 'Me',
-      supportedLanguages: ['en', 'ru'],
-      serviceLanguage: '',
     });
   });
 
@@ -411,7 +408,7 @@ describe('codec', () => {
     b.finish(fb.LinkResult.endLinkResult(b));
     const r = decodeLinkResult(b.asUint8Array());
     expect(r.status).toBe('merged');
-    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya', supportedLanguages: [], serviceLanguage: '' });
+    expect(r.session).toEqual({ token: 'tok-9', userId: 'a-1', isGuest: false, displayName: 'Kaya' });
   });
 
   it('decodes an Invitation with inviter and invitees', () => {
@@ -568,6 +565,35 @@ describe('codec', () => {
     b.finish(fb.Profile.endProfile(b));
     expect(decodeProfile(b.asUint8Array()).banner).toBeUndefined();
   });
+
+  it('decodes the profile variant preferences', () => {
+    const b = new Builder(64);
+    const uid = b.createString('u-1');
+    const prefs = fb.Profile.createVariantPreferencesVector(b, [b.createString('erudit_ru'), b.createString('scrabble_en')]);
+    fb.Profile.startProfile(b);
+    fb.Profile.addUserId(b, uid);
+    fb.Profile.addVariantPreferences(b, prefs);
+    b.finish(fb.Profile.endProfile(b));
+    expect(decodeProfile(b.asUint8Array()).variantPreferences).toEqual(['erudit_ru', 'scrabble_en']);
+  });
+
+  it('encodes the update-profile variant preferences', () => {
+    const buf = encodeUpdateProfile({
+      displayName: 'Kaya',
+      preferredLanguage: 'ru',
+      timeZone: 'UTC',
+      awayStart: '00:00',
+      awayEnd: '07:00',
+      blockChat: false,
+      blockFriendRequests: false,
+      notificationsInAppOnly: true,
+      variantPreferences: ['erudit_ru', 'scrabble_ru'],
+    });
+    const t = fb.UpdateProfileRequest.getRootAsUpdateProfileRequest(new ByteBuffer(buf));
+    const out: string[] = [];
+    for (let i = 0; i < t.variantPreferencesLength(); i++) out.push(t.variantPreferences(i));
+    expect(out).toEqual(['erudit_ru', 'scrabble_ru']);
+  });
 });
 
 // The live play loop exchanges alphabet indices, mapped through the per-variant