Move navroute generation into usenav composable.

[rtibbles]

Summary

• Turns the navItems export of the useNav composable into a computed prop
• Does generate nav items inside the use nav composable, instead of inside the core menu option and bottom navigation bar components - exposes subroute href as href property

References

In support of #5488

Reviewer guidance

Test that the side nav, top nav bar, and bottom nav bar all behave as expected without regressions.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

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

• 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

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0.dev0_git.20241003154926.pex
Windows Installer (EXE)	kolibri-0.18.0.dev0+git.20241003154926-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0.dev0+git.20241003154926-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0.dev0+git.20241003154926.dmg
Android Package (APK)	kolibri-0.18.0.dev0+git.20241003154926-0.1.4-debug.apk
TAR file	kolibri-0.18.0.dev0+git.20241003154926.tar.gz
WHL file	kolibri-0.18.0.dev0+git.20241003154926-py2.py3-none-any.whl

[marcellamaki]

code review looks good and manual QA checks out on main nav, side nav, and app bottom nav, including cross-plugin navigation into sub-routes