Add TOTP-based Multi-Factor Authentication UI

[tomascohen]

This adds the Angular frontend for TOTP Multi-Factor Authentication.

Companion to DSpace/DSpace#12703 which implements the backend REST API
and JWT-based two-phase authentication flow.

Implementation overview

• NgRx actions/effects/reducer extensions for MFA state (mfaRequired, mfaVerifying, mfaError)
• MfaService for HTTP calls to backend MFA endpoints
• Login flow modification: authenticateSuccess$ effect detects mfa_verified=false in JWT claims
• LogInMfaComponent: TOTP code and recovery code input during login
• ProfilePageMfaFormComponent: Full MFA lifecycle management (setup with QR provisioning URI, verify, disable, regenerate recovery codes)
• i18n keys for all MFA-related strings

User-facing changes

1. Login page shows TOTP verification input after successful password authentication (when MFA enabled)
2. Profile page gains a 'Two-Factor Authentication' card for setup/management
3. Recovery code input available as fallback during login

Dependencies

• Backend: Add TOTP-based Multi-Factor Authentication support DSpace#12703