🎨 Palette: Add loading state to authentication forms

[singhaditya21]

💡 What: Added visual loading state ("Signing in... ⏳" / "Signing up... ⏳") and disabled state to the authentication submit buttons while processing.
🎯 Why: Without visual feedback on the button, users might double-click or think the app has frozen during network latency.
📸 Before/After: Before, the button remained interactive with original text. After, the button is disabled and text updates to indicate processing.
♿ Accessibility: Improved by explicitly disabling the button, preventing multiple submissions and indicating to assistive technologies that the state is changing.

---

PR created automatically by Jules for task 6878447189603456215 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.