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/lib/variants.test.ts

@@ -4,6 +4,7 @@ import {
   ALL_VARIANTS,
   DEFAULT_VARIANTS,
   availableVariants,
+  showRegisterHint,
   supportsMultipleWordsToggle,
   multipleWordsForRequest,
   usesStarBlank,
@@ -38,6 +39,18 @@ describe('availableVariants', () => {
   });
 });
 
+describe('showRegisterHint', () => {
+  it('invites only an online guest holding a single variant', () => {
+    expect(showRegisterHint(true, 1, false)).toBe(true);
+  });
+
+  it('stays hidden for a registered player, offline, or when several variants are offered', () => {
+    expect(showRegisterHint(false, 1, false)).toBe(false);
+    expect(showRegisterHint(true, 1, true)).toBe(false);
+    expect(showRegisterHint(true, 2, false)).toBe(false);
+  });
+});
+
 describe('supportsMultipleWordsToggle', () => {
   it('is true for Russian variants only', () => {
     expect(supportsMultipleWordsToggle('scrabble_ru')).toBe(true);