fix(account): seed the email account display name from the local part

An email account was provisioned with no display name (unlike Telegram/VK, which
seed one), so an email login showed an empty name. Seed it from the email's local
part (before '@', trimmed and capped to the column width) on first contact; the
user can rename it later. Only new accounts are seeded — an existing account's name
is never overwritten.

backend/internal/inttest/email_test.go

@@ -373,3 +373,24 @@ func TestConfirmByTokenLinkMerge(t *testing.T) {
 		t.Fatalf("merge result = %+v, want NeedsMerge with owner=%s", res, owner)
 	}
 }
+
+// TestEmailAccountSeedsDisplayName seeds a new email account's display name from the
+// email's local part, so an email login is not left nameless.
+func TestEmailAccountSeedsDisplayName(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+	svc := account.NewEmailService(store, &capturingMailer{}, "https://erudit-game.ru")
+	local := "kaya-" + uuid.NewString()[:8]
+
+	id, err := svc.RequestLoginCode(ctx, local+"@example.com", "", "en")
+	if err != nil {
+		t.Fatalf("request login: %v", err)
+	}
+	acc, err := store.GetByID(ctx, id)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if acc.DisplayName != local {
+		t.Errorf("display name = %q, want the email local part %q", acc.DisplayName, local)
+	}
+}