🎨 Palette: Add visual loading states to auth buttons

[singhaditya21]

💡 What: Added "⏳ Loading..." visual states and disabled behavior to the Login, Register, and Google Sign-In buttons while their background requests process. State restoration is safely wrapped in try/finally blocks to guarantee recovery if errors occur.
🎯 Why: Users lacked visual feedback during authentication, leaving it unclear if their click registered and risking duplicate, frustrating submission attempts.
📸 Before/After: The button now visually disables and informs the user it is loading instead of remaining statically enabled during the simulated network wait.
♿ Accessibility: Disabling the button prevents screen reader users from accidentally double-triggering actions and communicates that the form is busy processing.

---

PR created automatically by Jules for task 5160368276102937280 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.