🎨 Palette: Add accessible loading states to auth forms

[singhaditya21]

💡 What: Added robust, accessible loading states to the login and registration form buttons.
🎯 Why: When users submit an auth form, there is no immediate visual feedback during the API request. Adding an hourglass indicator, loading text, and disabling the button improves clarity and prevents duplicate submissions.
📸 Before/After: See recorded verification video/screenshot.
♿ Accessibility: Added aria-busy="true" when the button is in a loading state. Used e.submitter to ensure the exact triggering element is handled properly, with robust cleanup in a finally block to prevent the UI from getting stuck if an error occurs. No custom CSS classes were added, preserving the existing design system.

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PR created automatically by Jules for task 6257861636343381443 started by @singhaditya21

[google-labs-jules]

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