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/account/link.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/go-jet/jet/v2/postgres"
 	"github.com/google/uuid"
+	"github.com/lib/pq"
 
 	"scrabble/backend/internal/postgres/jet/backend/table"
 )
@@ -282,9 +283,22 @@ func (s *Store) AttachIdentity(ctx context.Context, accountID uuid.UUID, kind, e
 // ClearGuest removes the is_guest flag from accountID, promoting an ephemeral guest
 // to a durable account once it gains its first identity. It is a no-op
 // for an already-durable account.
+//
+// The promotion also widens the guest's narrow variant set (GuestVariantPreferences,
+// Эрудит alone) to the registered default, so a player who registers from the New Game
+// hint really does gain Russian Scrabble. A guest who changed the set themselves is left
+// alone: only the untouched guest default is replaced.
 func (s *Store) ClearGuest(ctx context.Context, accountID uuid.UUID) error {
-	upd := table.Accounts.UPDATE(table.Accounts.IsGuest, table.Accounts.UpdatedAt).
-		SET(postgres.Bool(false), postgres.TimestampzT(time.Now().UTC())).
+	upd := table.Accounts.UPDATE(table.Accounts.IsGuest, table.Accounts.VariantPreferences, table.Accounts.UpdatedAt).
+		SET(postgres.Bool(false),
+			postgres.Raw(
+				"CASE WHEN variant_preferences = #guest_variants::text[] THEN #default_variants::text[] ELSE variant_preferences END",
+				map[string]interface{}{
+					"#guest_variants":   pq.StringArray(GuestVariantPreferences),
+					"#default_variants": pq.StringArray(DefaultVariantPreferences),
+				},
+			),
+			postgres.TimestampzT(time.Now().UTC())).
 		WHERE(
 			table.Accounts.AccountID.EQ(postgres.UUID(accountID)).
 				AND(table.Accounts.IsGuest.EQ(postgres.Bool(true))),