[FE-24] Build two-factor authentication setup page

[yusuftomilola]

Overview

Build a page where users can enable 2FA (TOTP) for their account: scan QR code, enter verification code, save backup codes. All work goes in frontend/sandbox/.

Acceptance Criteria

• Page at frontend/sandbox/settings/2fa/page.tsx
• Step 1: Show QR code (fetched from API) with instructions to scan with authenticator app
• Step 2: Input field to enter 6-digit TOTP code for verification
• Step 3: Display backup codes in a copyable list with download button
• Each step only visible after previous step is complete
• "2FA is enabled" success state shown after setup
• Disable 2FA option (requires current TOTP code)
• Uses mock data if no API connected