🎨 Palette: Add loading states to auth forms

[singhaditya21]

💡 What: Added visual loading states to the submit buttons on the login and register forms by disabling them and changing the text during the async API call.
🎯 Why: Missing loading states for async operations can lead to user confusion about whether their action registered, and can result in multiple unintended form submissions.
📸 Before/After: The button previously did not respond until the request finished. Now, it displays 'Logging in...' or 'Creating account...' while disabled.
♿ Accessibility: The disabled state on the button prevents double clicks and signals to assistive technology that the form is processing.

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PR created automatically by Jules for task 1230229725728691247 started by @singhaditya21

[google-labs-jules]

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