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/Profile.svelte

@@ -16,6 +16,8 @@
     validDisplayName,
     validEmail,
   } from '../lib/profileValidation';
+  import { ALL_VARIANTS, VARIANT_RULES } from '../lib/variants';
+  import type { Variant } from '../lib/model';
 
   let dn = $state('');
   let tz = $state('+00:00');
@@ -28,6 +30,7 @@
   let blockChat = $state(false);
   let blockFriendRequests = $state(false);
   let notificationsInAppOnly = $state(true);
+  let variantPrefs = $state<Variant[]>([]);
   let emailInput = $state('');
   let codeInput = $state('');
   let emailSent = $state(false);
@@ -60,6 +63,7 @@
     blockChat = p.blockChat;
     blockFriendRequests = p.blockFriendRequests;
     notificationsInAppOnly = p.notificationsInAppOnly;
+    variantPrefs = [...p.variantPreferences];
   }
   onMount(populate);
 
@@ -67,9 +71,19 @@
   const awayEnd = $derived(`${endH}:${endM}`);
   const nameOk = $derived(validDisplayName(dn));
   const awayOk = $derived(awayDurationOk(awayStart, awayEnd));
-  const formValid = $derived(nameOk && awayOk);
+  const formValid = $derived(nameOk && awayOk && variantPrefs.length >= 1);
   const emailOk = $derived(validEmail(emailInput));
 
+  // toggleVariant flips a variant in the preference set, refusing to remove the last one —
+  // the player must allow themselves at least one variant.
+  function toggleVariant(id: Variant) {
+    if (variantPrefs.includes(id)) {
+      if (variantPrefs.length > 1) variantPrefs = variantPrefs.filter((v) => v !== id);
+    } else {
+      variantPrefs = [...variantPrefs, id];
+    }
+  }
+
   async function save() {
     if (!formValid) return;
     try {
@@ -82,6 +96,7 @@
         blockChat,
         blockFriendRequests,
         notificationsInAppOnly,
+        variantPreferences: variantPrefs,
       });
       showToast(t('profile.saved'));
     } catch (e) {
@@ -207,6 +222,22 @@
           <input type="checkbox" bind:checked={notificationsInAppOnly} />
           <span>{t('profile.notificationsInAppOnly')}</span>
         </label>
+        <fieldset class="prefs">
+          <legend>{t('profile.preferences')}</legend>
+          <p class="muted">{t('profile.preferencesHint')}</p>
+          {#each ALL_VARIANTS as v (v.id)}
+            <label class="check pref">
+              <input
+                type="checkbox"
+                checked={variantPrefs.includes(v.id)}
+                disabled={variantPrefs.length === 1 && variantPrefs.includes(v.id)}
+                onchange={() => toggleVariant(v.id)}
+              />
+              <span class="pname">{t(v.label)}</span>
+              <span class="prule">{t(VARIANT_RULES[v.id])}</span>
+            </label>
+          {/each}
+        </fieldset>
         <div class="formacts">
           <button type="submit" class="btn" disabled={!formValid || !connection.online}>{t('common.save')}</button>
         </div>
@@ -342,6 +373,30 @@
     gap: 10px !important;
     color: var(--text) !important;
   }
+  .prefs {
+    border: 1px solid var(--border);
+    border-radius: var(--radius-sm);
+    padding: 10px 12px;
+    margin: 0;
+    display: flex;
+    flex-direction: column;
+    gap: 8px;
+  }
+  .prefs legend {
+    color: var(--text-muted);
+    font-size: 0.9rem;
+    padding: 0 4px;
+  }
+  .pref {
+    align-items: baseline;
+  }
+  .pname {
+    font-weight: 600;
+  }
+  .prule {
+    color: var(--text-muted);
+    font-size: 0.8rem;
+  }
   .formacts {
     display: flex;
     gap: 10px;