🎨 Palette: Add async loading states to authentication buttons

[singhaditya21]

What:
Added asynchronous loading states (text change, disabled status, and aria-busy) to the main authentication buttons (Login, Register, and Google Sign-in).

Why:
To provide immediate visual feedback to the user during authentication processes and prevent duplicate submissions or accidental double-clicks, significantly enhancing perceived performance and interaction reliability.

Before/After:

• Before: Clicking login simply triggered a toast without affecting the button itself.
• After: The button visibly changes to a disabled state with "⏳ Logging in..." (or appropriate text) while the operation is pending, and restores reliably even if an error occurs.

Accessibility:
Added aria-busy="true" and disabled attributes during active network requests to effectively notify screen reader users of the ongoing process and block extraneous interactions.

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PR created automatically by Jules for task 12828390507278886262 started by @singhaditya21

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