feat(account): give a fresh guest a distinct "Guest######" display name

Every guest showed a bare "Guest" to opponents (e.g. in a random match). Mint the display name
as "Guest" + a random six-digit suffix at provisioning, so guests are distinguishable. Not an
identifier and not unique — a label only, so collisions are harmless.

backend/internal/inttest/account_test.go

@@ -5,6 +5,7 @@ package inttest
 import (
 	"context"
 	"errors"
+	"regexp"
 	"testing"
 	"time"
 
@@ -241,6 +242,10 @@ func TestProvisionSeedsTimeZone(t *testing.T) {
 	if guest.PreferredLanguage != "ru" {
 		t.Errorf("guest PreferredLanguage = %q, want the seeded ru", guest.PreferredLanguage)
 	}
+	// A guest's display name is "Guest" + a random six-digit suffix (distinct to opponents).
+	if m, _ := regexp.MatchString(`^Guest\d{6}$`, guest.DisplayName); !m {
+		t.Errorf("guest DisplayName = %q, want Guest + 6 digits", guest.DisplayName)
+	}
 	plainGuest, err := store.ProvisionGuest(ctx, "", "")
 	if err != nil {
 		t.Fatalf("provision plain guest: %v", err)