🎨 Palette: Add loading states to authentication buttons

[singhaditya21]

💡 What

Added visual loading states to the Login, Registration, and Google Sign-in buttons. During form submission or authentication flows, the buttons are disabled and their text changes to "⏳ Loading...". The original state is reliably restored after the operation finishes using a try/finally block.

🎯 Why

Missing loading states on async operations lead to user confusion (e.g., wondering if the app registered their click) and can result in duplicate form submissions if users repeatedly click the button.

📸 Before/After

Before: Clicking "Sign In" provided no immediate visual feedback on the button itself while the network request resolved.
After: Clicking "Sign In" immediately disables the button and updates the text to indicate progress.

♿ Accessibility

• Disabling the button (disabled=true) prevents multiple screen reader announcements for repeated focus/clicks.
• The state natively restricts keyboard activation (Enter/Space) while loading.

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PR created automatically by Jules for task 12700262907118727654 started by @singhaditya21

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