🎨 Palette: Add loading states to authentication buttons

[singhaditya21]

💡 What

Added async loading states (disabled, opacity change, and ⏳ Loading... text) to the Login, Register, and Google Sign-in buttons.

🎯 Why

During network latency, users could spam the submit button, leading to duplicate submissions or confusion. These loading states provide immediate visual feedback that their action is processing and disables the button to prevent unintended multiple API calls.

📸 Before/After

(Verified via Playwright Screenshot capture of the "⏳ Loading..." state)

♿ Accessibility

• Buttons are explicitly set to disabled=true and cursor: not-allowed while the API call is inflight to prevent confusing keyboard/screen reader focus interactions on an inactive element.
• try...finally blocks strictly ensure state is restored exactly to the originalHtml in both success and error paths.

---

PR created automatically by Jules for task 4165394286025492640 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.