feat: add optional TOTP MFA for email/password accounts

- Add MFA enrollment UI in new Settings page with QR code and backup secret
- Add TOTP challenge step to login flow for enrolled users
- Check AAL after login and show TOTP input when aal2 required
- Add disable MFA option with TOTP re-verification
- Only show MFA options for email/password users (not OAuth)
- Add Settings link to user dropdown menu

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/index.ts

@@ -8,5 +8,6 @@ export { NewGenlocke } from './NewGenlocke'
 export { NewRun } from './NewRun'
 export { RunList } from './RunList'
 export { RunEncounters } from './RunEncounters'
+export { Settings } from './Settings'
 export { Signup } from './Signup'
 export { Stats } from './Stats'