🎨 Palette: Add async loading states to auth buttons

[singhaditya21]

💡 What: Added visual loading feedback to the "Sign In" and "Sign Up" buttons during async API calls.
🎯 Why: Without visual feedback, users don't know if their click registered, which can lead to confusion and double-submissions on form forms.
📸 Before/After: Before: The button remained static during login/registration delays. After: The button becomes disabled and its text updates to "Signing In..." or "Signing Up..." while the network request is in flight.
♿ Accessibility: Added a disabled attribute during the async process, preventing screen readers and keyboard users from accidentally submitting the form multiple times.

---

PR created automatically by Jules for task 11315449514050512924 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.