FEATURE REQUEST: Add back button, fix Resend cooldown state, and improve OTP input UX

[g-k-s-03]

Feature Description
Improve usability of the OTP verification screen.

Problem Statement

• VerifyOTPScreen has no back/cancel button — a user who wants to leave has no in-app way to do so.
• The Resend button remains visually enabled during its 60s cooldown; tapping it silently no-ops instead of being disabled with clear feedback.
• The 6-box OTP input has no backspace-to-previous-box behavior and no paste support for a copied code.

Expected Outcome

• Add a Back/Cancel action consistent with ForgotPasswordScreen/SetNewPasswordScreen.
• Gate the Resend button's onPressed on the cooldown flag, not just loading state.
• Support backspace navigating to the previous box, and pasting a full code auto-filling all boxes.

Additional Context
Affects: lib/screens/auth/verify_otp_screen.dart.

Record

• I agree to follow this project's Code of Conduct
• I want to work on implementing this feature