🎨 Palette: Add loading states to authentication buttons

[singhaditya21]

What

Added visual loading states (disabling the button, showing "⏳ Loading...", and lowering opacity) to all authentication forms (Login, Register, and Google Sign-In).

Why

Users often lack feedback during asynchronous authentication operations. Immediate visual feedback reduces uncertainty and prevents accidental duplicate form submissions.

Before/After

Before: Clicking login/register provided no immediate feedback on the button itself while the API request was pending.
After: The button disables and shows a loading state immediately upon click, restoring to its original state when the request completes.

Accessibility

Prevents keyboard users from accidentally submitting the form multiple times by properly leveraging the disabled attribute during the async operation.

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PR created automatically by Jules for task 5119929961480502216 started by @singhaditya21

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