Update Verify OTP page to new design system

[yusuftomilola]

Description:
Update the Verify OTP page to use the new design system colors. Replace blue backgrounds, text colors, focus rings, and borders with gray equivalents. Update background to bg-[#faf9f7]. Keep OTP input logic, verify, and resend functionality unchanged.

Acceptance Criteria:

• Background uses bg-[#faf9f7]
• Buttons use bg-gray-900 / hover:bg-gray-800
• OTP input focus uses focus:border-gray-900 and focus:ring-gray-300
• No functional changes to OTP logic

File(s): frontend/app/(auth)/verify-otp/page.tsx