Add z-index to fix display of HybridLearningFooter dropdown menu

[LianaHarris360]

Summary

• Updates the CoreMenu styling within the HybridLearningFooter component to include z-index: 7 so that the menu displays on top of other cards, instead of behind.

Before:

After:

References

Fixes #11378
The functionality of the 'Remove from my library' option is fixed in PR #11397

Reviewer guidance

1. Go to Explore libraries and click the download icon on a card.
2. Observe the position of 'Remove from my library'.

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[rtibbles]

Code changes make sense to me - not sure how else we would achieve this except with the z-index tweak!

[rtibbles]

@pcenov when testing this, note that this PR fixes the proper display of the button, but not its functionality - that is fixed in #11397 as @LianaHarris360 notes.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b6.dev0_git.118.g361c99bc.pex
Windows Installer (EXE)	kolibri-0.16.0b6.dev0+git.118.g361c99bc-unsigned.exe
Debian Package	kolibri_0.16.0b6.dev0+git.118.g361c99bc-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b6.dev0+git.118.g361c99bc-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b6.dev0+git.118.g361c99bc-0.1.0-debug.apk
TAR file	kolibri-0.16.0b6.dev0+git.118.g361c99bc.tar.gz
WHL file	kolibri-0.16.0b6.dev0+git.118.g361c99bc-py2.py3-none-any.whl

[pcenov]

LGTM - implemented as specified! Will further test some scenarios once #11397 is finalized and merged.