Add OTP Verification for Signup

Description: Currently, signup only asks for email, password, and username. Add a phone number field and verify it using OTP.

Acceptance Criteria:

• Users must enter a valid phone number during signup.
• Send an OTP to the entered number to verify it.
• Users cannot complete signup until OTP is verified.
• Show an error message if the number is invalid or OTP fails.
• Store verified phone number in the user database.
• Allow resending OTP.
• Limit OTP attempts to prevent abuse.