🎨 Palette: Improve Login accessibility and add loading states

[BenjaminWie]

Improved the user experience and accessibility of the login process and main interface.

Key changes:

1. Login Flow Overhaul: Updated `App.tsx` and `LoginView.tsx` to handle login success/failure through component state instead of blocking `alert()` calls.
2. Interactive Feedback: Added a loading state and spinner to the login button to provide immediate feedback during the authentication check.
3. Accessibility (a11y) Enhancements:
   • Added a screen-reader-only label for the email input.
   • Implemented `aria-invalid` and `aria-describedby` for robust error reporting.
   • Added `aria-label` attributes to icon-only buttons in `ChatView` (Voice, Send) and `Sidebar` (Close) to ensure they are correctly identified by screen readers.
4. Visual Polish: Integrated a new `LoadingIcon` based on Lucide's `Loader2` for consistent UI feedback.

All changes were verified via `pnpm build` and automated Playwright scripts.

---

PR created automatically by Jules for task 15099980824869587676 started by @BenjaminWie

[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.