Add User

Logic:

• Extract required fields (username, password, full name, birthday) from the POST request.

• Extract optional fields (email, avatar) using request.form.get.

• Hash the plain text password into an SHA-1 string and save it in the JSON.

• Write the new data to the JSON file.

Frontend:

• Four mandatory fields: username (for login), password, full name, birthday (not currently used).

• Optional email field (used for email notifications).

• Avatar selection: choose from six avatars, replace files in static/icons with appropriately named PNG files (avatar1.png, avatar2.png, etc.) if you want different ones.