feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

ui/src/screens/NewGame.svelte

@@ -22,6 +22,7 @@
   import type { AccountRef, GameView, Variant } from '../lib/model';
   import {
     availableVariants,
+    showRegisterHint,
     VARIANT_FLAG,
     VARIANT_RULES,
     supportsMultipleWordsToggle,
@@ -395,6 +396,13 @@
             </button>
           {/each}
         </div>
+        <!-- A guest is offered Erudit alone; registering widens the account to both Russian games
+             (and Settings then offers English), so point them at the profile screen. -->
+        {#if showRegisterHint(guest, variants.length, offlineMode.active)}
+          <p class="guesthint">
+            <button class="hintlink" onclick={() => navigate('/profile')}>{t('new.guestVariantHintLink')}</button>{t('new.guestVariantHint')}
+          </p>
+        {/if}
         {#if variants.some((v) => supportsMultipleWordsToggle(v.id))}
           <label class="toggle">
             <span>{t('new.multipleWordsPerTurn')}</span>
@@ -771,6 +779,24 @@
     font-size: 0.85rem;
     line-height: 1.4;
   }
+  /* The guest's register invitation: a muted line opening with an inline link-styled button
+     that routes to the profile screen (mirrors Profile's .linklike). */
+  .guesthint {
+    margin: 0;
+    text-align: center;
+    color: var(--text-muted);
+    font-size: 0.85rem;
+    line-height: 1.4;
+  }
+  .hintlink {
+    background: none;
+    border: none;
+    padding: 0;
+    color: var(--accent);
+    font: inherit;
+    text-decoration: underline;
+    cursor: pointer;
+  }
   /* The offline-blocker reason (a missing dictionary, or the online segment needing a connection). */
   .dictnote {
     margin: 0;