[Plugin] WebAuthn plugin — Portal UI integration

[maxzaikin]

Summary

Add WebAuthn enrollment and verification UI to the Blazor Portal.
Requires JavaScript interop for WebAuthn browser API calls.

Context

See API-MFA — WebAuthn section

Acceptance Criteria

• Settings → MFA → Add Security Key button (shown when plugin active)
• Device name input field before enrollment begins
• JavaScript interop: navigator.credentials.create() for enrollment
• JavaScript interop: navigator.credentials.get() for verification
• Enrollment success screen with device name confirmation
• Enrolled devices list: name, enrolled date, last used, revoke button
• WebAuthn verification step in checkout MFA flow
• Browser compatibility check (show warning if WebAuthn not supported)
• Plugin unavailable → graceful error in UI