Feat/custom icon upload

This adds the option to upload a custom icon and would fix #145. Screenshot of the edit page: <img width="1130" height="961" alt="image" src="https://github.com/user-attachments/assets/6379f644-dfc7-4341-92df-b05f9a8078e4" /> Screenshot of the login page: <img width="779" height="567" alt="image" src="https://github.com/user-attachments/assets/4c5af9ca-a21e-4caf-b5a4-f201c1ed8c4f" />