Add TOTP-based two-factor authentication

- Add 2FA setup, enable, disable, and backup code management
- Integrate 2FA challenge flow into login process
- Add frontend modal for 2FA configuration
- Support backup codes for account recovery

frontend/src/api/backend/index.ts

@@ -60,3 +60,4 @@ export * from "./updateStream";
 export * from "./updateUser";
 export * from "./uploadCertificate";
 export * from "./validateCertificate";
+export * from "./twoFactor";