🎨 Palette: Add async loading states to auth buttons

[singhaditya21]

💡 What: Added asynchronous loading states to the authentication buttons (Login, Register, and Google Sign-in) on the auth screens.
🎯 Why: Without visual feedback, users might click the button multiple times while waiting for the request to complete, leading to confusion or duplicate submissions. This improves the perceived performance and clarity.
📸 Before/After: Before, buttons remained active and static during submission. After, they visually update to indicate "Logging in..." or "Signing in..." and are temporarily disabled.
♿ Accessibility: Added aria-busy="true" and disabled=true during the loading phase to ensure screen reader users are aware of the processing state and cannot re-trigger the action.

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PR created automatically by Jules for task 5544968488616132957 started by @singhaditya21

[google-labs-jules]

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