🎨 Palette: Add async loading states to authentication forms

[singhaditya21]

💡 What: Added visual loading spinners (aria-busy="true", disabled="true") to the Login and Registration form submit buttons while waiting for the mock API response.
🎯 Why: Without visual feedback during asynchronous API calls, users might think the application is frozen or unresponsive, leading them to click the submit button multiple times or abandon the task.
📸 Before/After: Visual changes verified via Playwright screenshot; the button displays an hourglass and "Logging in..." / "Signing up..." text during request.
♿ Accessibility: The loading state includes an aria-busy="true" attribute to notify screen readers of the pending state, the visual emoji is hidden from screen readers via aria-hidden="true", and the button is disabled to prevent duplicate submissions via both pointer and keyboard actions. The state updates are safely wrapped in a finally block to ensure they reset if an error occurs.

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PR created automatically by Jules for task 8222680433597006948 started by @singhaditya21

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