🎨 Palette: Add loading state to async buttons

[singhaditya21]

💡 What: Added loading state and disabled the submit button during async authentication actions (login, register, and google sign-in).
🎯 Why: To provide clear visual feedback to the user and prevent duplicate submissions while the action is in progress.
📸 Before/After: Before, only a toast message was shown and the button remained active. After, the button text changes to "Signing In..." / "Signing Up..." and gets disabled with a lower opacity.
♿ Accessibility: Improved keyboard/assistive technology feedback by disabling the button and changing its text, preventing multiple click errors.

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PR created automatically by Jules for task 11249437455744489152 started by @singhaditya21

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