🎨 Palette: Add Escape key and ARIA roles to modals

[jspann21]

💡 What: Added Escape key handlers and dialog ARIA roles to remaining unstandardized modals (CoverPickerModal, MatchPreviewModal, RescanMetadataModal).
🎯 Why: To standardize keyboard accessibility and screen reader support across all dialogs.
📸 Before/After: Visual changes unnoticeable, DOM changes provided in diff.
♿ Accessibility: Ensures users can easily close dialogs with Escape and screen readers announce them properly as modals.

---

PR created automatically by Jules for task 1370686382789099406 started by @jspann21

[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.