Implement Custom Alert Page for Silent MFA Prompt on Checkmk Login

[kayiwa]

Description:
Users currently receive a silent Multi-Factor Authentication (MFA) prompt (e.g., via push notification) during Checkmk login but are not explicitly informed about it. This can cause confusion, especially if users:

1. Are unaware their MFA method is enabled.
2. Do not realize they need to check their authenticator app/device.
3. Mistake the delay for a system error.

Proposed Solution:
Create a customized interstitial page that:

• Clearly informs users they will receive an MFA prompt.
• Provides brief instructions (e.g., "Check your authenticator app or device to approve the login").
• Includes troubleshooting tips (e.g., resend prompt, alternate methods).

Acceptance Criteria:

1. Page displays after initial username/password validation and before the MFA check.
2. Clean, user-friendly design matching Checkmk’s branding.
3. Includes:

• A clear message (e.g., "Check your device for an MFA approval prompt").