Fixes numeric keypad z-index value

[AlexVelezLl]

Summary

• Updates the numeric keypad z-index to 10, which follows the KDS z-index guidelines, and it lies on top of the BottomAppBar (z-index: 8, but later in the DOM) and below the Side Panel backdrop (z-index: 11).
• Checked through different scenarios hunting bugs, but found nothing.

References

Closes #15158.

Reviewer guidance

• Open an exercise resource with a numeric input.
• Click on the input.
• Play with different screen sizes, different scenarios, and check that the position of the NumericKeypad is correct.

AI usage

None.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.20.0a2.dev248_gcf12e09bf.pex
Windows Installer (EXE)	kolibri-setup-0.20.0a2.dev248+gcf12e09bf-unsigned.exe
Debian Package	kolibri_0.20.0a2.dev248+gcf12e09bf-0ubuntu1_all.deb
Debian Package	kolibri-server_0.20.0~a2.dev248+gcf12e09bf-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.20.0a2.dev248+gcf12e09bf.dmg
Android Package (APK)	kolibri-0.20.0a2.dev248+gcf12e09bf-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.20.0a2.dev248+gcf12e09bf.zip
TAR file	kolibri-0.20.0a2.dev248+gcf12e09bf.tar.gz
WHL file	kolibri-0.20.0a2.dev248+gcf12e09bf-py3-none-any.whl

Smoke test screenshot

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