refactor: move gateway and vault packages to internal directory

internal/gateway/handlers/login_handler.go

@@ -0,0 +1,24 @@
+package handlers
+
+import "github.com/labstack/echo/v4"
+
+func HandleLogin(c echo.Context) error {
+	return nil
+}
+
+//
+// func LoginHandler(c echo.Context) error {
+// 	options := PublicKeyCredentialRequestOptions{
+// 		Challenge: "your-challenge-base64url",
+// 		RpID:      "yourdomain.com",
+// 		Timeout:   60000,
+// 		AllowCredentials: []CredentialDescriptor{
+// 			{
+// 				Type: "public-key",
+// 				ID:   "credential-id-base64url",
+// 			},
+// 		},
+// 	}
+//
+// 	return GetCredentials(c, options)
+// }