🎨 Palette: Add loading states to auth buttons

[singhaditya21]

💡 What

Added explicit visual loading states to all authentication buttons (Login, Register, and Google Sign-In). When clicked, the buttons now show a "⏳ Loading..." state, become disabled, and slightly dim to indicate an active process.

🎯 Why

During asynchronous UI interactions (like form submissions or OAuth redirects), lack of immediate feedback can lead users to believe the application is frozen or unresponsive, often resulting in repeated clicks. By providing explicit loading states, we improve perceived performance and prevent duplicate submissions.

📸 Before/After

Before: Clicking login/register/google sign-in showed no visual change on the button itself while waiting for the network request.
After: The button content changes to "⏳ Loading...", it becomes disabled (unclickable), and its opacity drops to 0.7 until the request completes.

♿ Accessibility

• Prevents redundant form submissions by correctly utilizing the HTML disabled attribute on active buttons.
• Keeps focus on the disabled element appropriately in modern browsers, ensuring screen readers announce the state change correctly without disorienting the user.
• Utilizes e.submitter to gracefully handle different form submission methods.

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PR created automatically by Jules for task 3040025987743897442 started by @singhaditya21

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