🎨 Palette: Add loading states to authentication buttons

[singhaditya21]

💡 What

Added asynchronous loading states to the login, register, and Google sign-in buttons in the web-demo app.

🎯 Why

Without a loading state, users may think the application has frozen during authentication, leading to frustration or multiple form submissions (double-clicking).

📸 Before/After

Before: Clicking "Sign In with Email" or "Sign Up" showed no visual change to the button, though a small toast would appear.
After: The button becomes disabled, its opacity drops to 70%, and the text updates to "Signing In..." (or "Signing Up..."), providing immediate visual feedback at the site of interaction.

♿ Accessibility

The disabled attribute is dynamically applied to the button during the network request. This prevents screen readers and keyboard users from accidentally triggering the action multiple times.

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PR created automatically by Jules for task 10127016804090492668 started by @singhaditya21

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