🎨 Palette: Add loading states to authentication buttons

[singhaditya21]

💡 What: Added dynamic loading text ("Signing In...", "Signing Up...") and disabled states (reduced opacity) to the primary authentication buttons during login, registration, and Google Sign-In flows.

🎯 Why: Previously, the authentication buttons lacked visual feedback during asynchronous network requests. This could lead to user confusion (wondering if the click registered) and potential double-submissions, degrading the UX.

📸 Before/After:
(See verification screenshots - login buttons now dynamically update text and disable during API calls)

♿ Accessibility:

• Adding the disabled property explicitly signals to screen readers and keyboard users that the button is currently inactive and processing, preventing sequential erroneous focus actions.
• Reduces cognitive load by providing immediate visual acknowledgment of user interaction.

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PR created automatically by Jules for task 8195217299176087336 started by @singhaditya21

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