🎨 Palette: Add visual feedback for interactive elements and accessibility

[kastnerp]

What:

• Added missing aria-label, aria-expanded, and focus-visible styling to the sidebar toggle buttons in src/App.svelte.
• Added disabled state visual feedback (disabled:opacity-50 disabled:cursor-not-allowed disabled:hover:bg-transparent) to the "Zoom In" and "Exclude" buttons in src/lib/components/layout/Navbar.svelte.

Why:

• The icon-only toggle buttons were previously unreadable by screen readers and lacked keyboard focus indicators.
• The disabled state of the "Zoom In" and "Exclude" buttons was functionally correct but visually identical to the enabled state, which could confuse users.

Before/After:

• Before: Sidebar buttons lacked screen reader context. Disabled state on Navbar buttons looked like clickable buttons.
• After: Sidebar buttons are fully accessible. Navbar buttons visibly appear disabled (greyed out, no cursor) when there's no selection.

Accessibility:

• Added aria-label and aria-expanded attributes.
• Added explicit focus-visible ring styling for keyboard navigation.

---

PR created automatically by Jules for task 11938319096792668672 started by @kastnerp

[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.