security: add auth session manager, input sanitizer, and password strength meter

[Babin123456]

Related Issue

Closes #421

Summary

Implements a unified Auth Session Manager, DOM XSS Security Input Sanitizer, and dynamic Password Strength Meter for Furnix account authentication flows.

Changes Made

• Created scripts/security-sanitizer.js for HTML escaping and password strength scoring.
• Created scripts/auth-manager.js consolidating login, registration, and logout logic.
• Updated login.html, signup.html, account.html, and style.css.
• Deleted fragmented legacy scripts (login-auth.js, signup-auth.js, account-auth.js) and duplicate Furnix-main/login.html.

Testing

• Tested XSS input payloads (<script>alert(1)</script>) to verify HTML escaping.
• Tested real-time password strength bar indicators on registration.

Impact

Improves security posture, reduces script fragmentation, and improves developer experience.

Checklist

• Code follows project standards
• Tested locally
• No unrelated changes included
• Responsive design verified
• Accessibility considered

[vercel]

@Babin123456 is attempting to deploy a commit to the janavipandole's projects Team on Vercel.

A member of the Team first needs to authorize it.