🎨 Palette: [UX improvement] Add loading spinner to auth button

[Amer-alsayed]

💡 What: Added a spinning loader icon (RefreshCw from lucide-react) to the authentication button when a login or registration request is in progress.
🎯 Why: Cryptographic operations on the client-side (such as key derivation) take some time, and clicking the button without immediate visual feedback can cause the user to doubt whether the app is responding or not. By adding an animated spinner, the UI provides immediate response to the click action, making the app feel significantly faster and more interactive while blocking further submissions.
📸 Before/After: Before, the button merely disabled itself and changed text to "Processing Cryptography...". After, it changes to an animated spinner accompanied by "Processing Cryptography..." in a nicely centered flex layout.
♿ Accessibility: The button was already properly labeled and disabled. The text is still readable, and the animated spinner adds an additional visual cue for users with cognitive or visual impairments relying on clear state transitions.

---

PR created automatically by Jules for task 7506266807631405915 started by @Amer-alsayed

[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.