Add rainbow effect to login page

[kc-beard]

Summary

• Added an animated rainbow gradient background to the login page with a smooth 8-second cycling animation
• Applied a white overlay to ensure the login form remains readable and accessible
• Uses CSS keyframe animations for smooth, performant visual effects

Test plan

• Navigate to the login page at /login
• Observe the animated rainbow gradient background transitioning smoothly through colors
• Verify the login form is readable with proper contrast against the background
• Test on different screen sizes to ensure responsive design
• Confirm login functionality is not affected by the new styling

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