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

**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

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on implementing this feature