🎨 Palette: Add Loading States to Auth Buttons

[singhaditya21]

💡 What: Added accessible visual loading states to the Login, Sign Up, and Google Sign-in buttons. The buttons now properly disable to prevent multi-clicks, slightly reduce opacity, and update text to indicate the processing state ("Logging in...", "Signing up...", etc).

🎯 Why: Users previously had no clear visual feedback when clicking auth buttons, leading to potential confusion or multiple rapid clicks while waiting for the async API or simulated OAuth redirect. This improves clarity and interaction feedback.

♿ Accessibility: The disabled attribute immediately informs screen readers that the form submission is processing and prevents duplicate focus/submission attempts.

Before/After: Visually verified via Playwright. Button text changes during delay, disabled attribute is set, opacity shifts to 0.7.

Additionally fixed a latent bug in handleGoogleSignIn by converting the setTimeout callback into an awaited Promise so that a try/finally block could cleanly and safely restore UI state across successes or failures.

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PR created automatically by Jules for task 12130418285211916238 started by @singhaditya21

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