🎨 Palette: Add loading state to login form

[singhaditya21]

💡 What: Added a visual loading state to the email login button during authentication.
🎯 Why: Users need immediate feedback when submitting a form, and the button should be disabled to prevent accidental double-submissions while the asynchronous API request completes.
📸 Before/After: Before, the button remained clickable with unchanged text during the API request. After, the button text updates to "⏳ Logging in...", the button is visibly faded, disabled from clicks, and receives aria-busy="true".
♿ Accessibility: Added aria-busy="true" to announce the loading state to screen readers, and enhanced the .btn class with a clear :focus-visible state for better keyboard navigation.

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PR created automatically by Jules for task 10284543296374206635 started by @singhaditya21

[google-labs-jules]

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