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.

backend/internal/inttest/variant_preference_test.go

@@ -16,10 +16,10 @@ import (
 
 // TestVariantPreferenceGate covers the New Game gate: a player may start a quick game
 // or create a friend invitation only in a variant they have enabled in their profile
-// (a fresh account defaults to Erudit only), and any other variant is refused with 400.
-// The complementary case — an invited friend accepting an invitation in a variant they
-// have NOT enabled — is exercised by TestGameLimitGate, where a default-Erudit human
-// accepts an English invitation over HTTP.
+// (a fresh account defaults to the two Russian-alphabet games), and any other variant is
+// refused with 400. The complementary case — an invited friend accepting an invitation in
+// a variant they have NOT enabled — is exercised by TestGameLimitGate, where a
+// default-preference human accepts an English invitation over HTTP.
 func TestVariantPreferenceGate(t *testing.T) {
 	clearOpenGames(t)
 	srv := server.New(":0", server.Deps{