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.

backend/internal/adminconsole/templates/pages/broadcast.gohtml

@@ -5,7 +5,6 @@
 {{if .ConnectorEnabled}}
 <form class="form col" method="post" action="/_gm/broadcast">
 <label>Message <textarea name="text" required></textarea></label>
-<label>Bot language <select name="language"><option value="en">en</option><option value="ru">ru</option></select></label>
 <div><button type="submit">Post to channel</button></div>
 </form>
 {{else}}<p class="note">connector not configured (set BACKEND_CONNECTOR_ADDR)</p>{{end}}

backend/internal/adminconsole/templates/pages/feedback_detail.gohtml

@@ -5,7 +5,7 @@
 <section class="panel"><h2>Message</h2>
 <ul class="kv">
 <li><b>From</b> <a href="/_gm/users/{{.AccountID}}">{{.SenderName}}</a> ({{.Source}})</li>
-<li><b>Channel</b> {{.Channel}}{{if .BotLanguage}} (bot: {{.BotLanguage}}){{end}}</li>
+<li><b>Channel</b> {{.Channel}}</li>
 <li><b>Interface language</b> {{.InterfaceLanguage}}</li>
 <li><b>IP</b> {{if .IP}}<code>{{.IP}}</code>{{else}}<span class="note">none</span>{{end}}</li>
 <li><b>Filed</b> {{.CreatedAt}}</li>

backend/internal/adminconsole/templates/pages/user_detail.gohtml

@@ -137,7 +137,6 @@
 {{if .ConnectorEnabled}}
 <form class="form col" method="post" action="/_gm/users/{{.ID}}/message">
 <label>Message <textarea name="text" required></textarea></label>
-<label>Bot language <select name="language"><option value="en">en</option><option value="ru">ru</option></select></label>
 <div><button type="submit">Send to user</button></div>
 </form>
 {{else}}<p class="note">connector not configured (set BACKEND_CONNECTOR_ADDR)</p>{{end}}