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/email_test.go

@@ -135,10 +135,11 @@ func TestUpdateProfilePersists(t *testing.T) {
 	store := account.NewStore(testDB)
 	acc := provisionAccount(t)
 
-	// A fresh account defaults to Erudit only (the DB-level column default).
+	// A fresh registered account defaults to both Russian-alphabet games (the DB-level
+	// column default); the lifecycle around it is covered by TestVariantPreferenceDefaults.
 	if def, err := store.GetByID(ctx, acc); err != nil {
 		t.Fatalf("get default: %v", err)
-	} else if want := []string{"erudit_ru"}; !slices.Equal(def.VariantPreferences, want) {
+	} else if want := account.DefaultVariantPreferences; !slices.Equal(def.VariantPreferences, want) {
 		t.Errorf("default variant preferences = %v, want %v", def.VariantPreferences, want)
 	}