i can build full app.I will make interface with flutter and backend with python.The app which i will make can run all platform but i will charge some money for it. I will take less time for making interface but for backend it depends that how many features you add. I am a ethical hacker also so if you want me to find vulnablitities then i can do that also but i will charge some money. I can make security algorithm which has to be use in order to make your app good with functionality and security wise both.