🎨 Palette: Add visual loading states to async form submissions

[singhaditya21]

💡 What

Added visual loading states to the async form submission buttons (login and register) using vanilla JavaScript. When the form is submitted, the button's text changes to "⏳ Loading...", it becomes disabled, and its opacity is reduced. Once the async API call completes (success or failure), the button's original state is restored.

🎯 Why

To improve the user experience by providing immediate feedback when an asynchronous action (like login or registration) is initiated. This prevents duplicate submissions by disabling the button and clarifies system state for the user.

📸 Before/After

• Before: When clicking submit, the button remains static, and the user might wonder if their click registered, potentially clicking multiple times.
• After: The button immediately reflects a loading state (⏳ Loading...) and is disabled.

♿ Accessibility

• Prevents redundant form submissions.
• Makes the current system status clear to users interacting with the form.

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PR created automatically by Jules for task 8657053407568052301 started by @singhaditya21

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