🎨 Palette: Add visual loading states to auth buttons

[singhaditya21]

💡 What:
Added a "⏳ Loading..." state to the Login, Register, and Google Sign-In buttons that triggers during authentication. The buttons are also disabled during this period to prevent double-submissions. The original text and state are safely restored in finally blocks.

🎯 Why:
Authentication can be slow (e.g. hitting external APIs or databases). Without a loading state, users often click the button multiple times thinking their input wasn't registered, which can lead to confusing errors or duplicate requests. This gives them immediate feedback that the system is working.

📸 Before/After:
Before: Clicking sign in does nothing visually to the button until the request completes.
After: Button text changes to "⏳ Loading..." and becomes disabled immediately on click.

♿ Accessibility:
Disabling the button explicitly communicates to assistive technologies that the control is currently unavailable, preventing focus confusion and clearly signaling the form's busy state.

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PR created automatically by Jules for task 1908159549572237409 started by @singhaditya21

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