feat: implement login page

[Chisomchima]

Implements DHIS2-18114

Description

This PR introduces a simple HTML login page that will be served by the backend as a backup for the current login system. This implementation serves as a fallback in case the main login system is unavailable. It uses plain HTML, CSS, and JavaScript for basic user authentication flow.

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Checklist

• Added an HTML file (backup-login.html) to provide a basic login interface with fields for username and password.
• The form uses simple JavaScript to handle form submission, sending credentials to the "/api/auth/login" endpoint via a POST request.
• Basic error handling is implemented to display an error message on invalid login attempts.

All points above should be relevant for feature PRs. For bugfixes, some points might not be relevant. In that case, just check them anyway to signal the work is done.

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Screenshots

[tomzemp]

Looks great! 🤩. It looks like it's working quite well. I think the most important thing is just to add the 2fa field in case users have that enabled.

[tomzemp]

lgtm 🚀

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud