fix(ui-shell): improve header global action a11y

[heloiselui]

Closes #19374

Continuation of #19289, that improves the behavior of header-global-action so the right panel now closes when focus is lost, and can also be toggled using Enter or Space. It also brings it in line with how the React version works.

Changelog

New

• Added support for toggling the panel using Enter and Space keys.
• Added focusout and document.click handlers to close the panel when focus is lost or the user clicks outside.

Changed

• Used @HostListener for keydown to match the keyboard interaction.
• Used ownerDocument instead of document to make sure the panel is found even inside Shadow DOM.

Testing / Reviewing

• Go to Web Components Deploy Preview > Header w/ Actions and Right Panel.
• Click or press **Enter or Space** on the **Notifications** button:
  • It should open the right panel.
  • Press **Enter or Space** again to close it.
• Click or tab away from the button/panel:
  • The panel should close automatically.

I used this as reference:

• https://lit.dev/docs/components/shadow-dom/
• https://web.dev/articles/shadowdom-v1
• https://developer.mozilla.org/en-US/docs/Web/API/Node/ownerDocument

PR Checklist

As the author of this PR, before marking ready for review, confirm you:

• Reviewed every line of the diff
• [N/A] Updated documentation and storybook examples
• [N/A] Wrote passing tests that cover this change
• Addressed any impact on accessibility (a11y)
• Tested for cross-browser consistency
• Validated that this code is ready for review and status checks should pass

More details can be found in the pull request guide

[netlify]

✅ Deploy Preview for v11-carbon-web-components ready!

Name	Link
🔨 Latest commit	a826eac
🔍 Latest deploy log	https://app.netlify.com/projects/v11-carbon-web-components/deploys/68384e0a26a5310009b3abc3
😎 Deploy Preview	https://deploy-preview-19509--v11-carbon-web-components.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[netlify]

✅ Deploy Preview for v11-carbon-react ready!

Name	Link
🔨 Latest commit	a826eac
🔍 Latest deploy log	https://app.netlify.com/projects/v11-carbon-react/deploys/68384e0ac421580008135563
😎 Deploy Preview	https://deploy-preview-19509--v11-carbon-react.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.48%. Comparing base (781e9d0) to head (a826eac).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19509      +/-   ##
==========================================
- Coverage   84.91%   84.48%   -0.44%     
==========================================
  Files         373      373              
  Lines       14504    14641     +137     
  Branches     4794     4837      +43     
==========================================
+ Hits        12316    12369      +53     
- Misses       2040     2125      +85     
+ Partials      148      147       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.