🎨 Palette: Add async loading states to auth buttons

[singhaditya21]

💡 What: Added visual loading states (disabled attribute and '⏳ Loading...' text) to the authentication buttons (Login, Register, and Google Sign-In) during asynchronous API calls. I also ensured that the button state is reliably restored in a finally block, refactoring the simulated Google Sign-In delay to use an awaitable Promise so the finally block executes at the correct time.

🎯 Why: When users click submit on a form or an auth button, lack of immediate visual feedback can lead to confusion and multiple clicks, which might trigger duplicate API requests or unexpected errors. Disabling the button and showing a loading indicator provides immediate confidence that the action is processing.

📸 Before/After: Before, the buttons stayed completely static when clicked, and users could repeatedly click them while the API requests were in flight. Now, the buttons instantly disable and show "⏳ Loading...", reverting to their original state automatically when the action concludes (even if an error occurs).

♿ Accessibility: Adding the disabled attribute to the button while processing prevents keyboard users from accidentally triggering multiple form submissions via repeated Enter keystrokes, enforcing a single, robust interaction loop.

---

PR created automatically by Jules for task 5871172521339284252 started by @singhaditya21

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.