Ensure loading indicator doesn't show outside it's context

[richard-cox]

Summary

Fixes #5245

Occurred changes and/or fixed issues

• Windows Manager sits below side nav and main screen content
  • shell, log streaming
• The relative and content loading indicator covers a specific area or the main screen content
• Bug - relative and content loading indicator appears over windows manager
• Fix
  • apply different z-index to main and other modes of loading indicator
  • assign a higher z-index to windows manager (higher than main screen content)

Technical notes summary

Tested

• mode - relative
  • shell/chart/monitoring/index.vue
  • shell/components/Wizard.vue
  • shell/components/nav/GlobalLoading.vue
  • shell/pages/auth/login.vue
• mode - content
  • shell/pages/c/_cluster/monitoring/index.vue
  • shell/edit/namespace.vue - shell/pages/c/_cluster/settings/banners.vue
• mode - main
  • not actually used... and currently broken (new burger menu is under it)
• outside content (such as burger menu, namespace filter, user avatar, etc) correctly shows above relative and content loading indicator
• wm drag and drop (working in vue3!)

Areas or cases that should be tested

This impacts places where we show the Loading... text in the centre of a opaque area. I've tested a lot above but via tweaking code. It would sense to maybe click around playing with the network tool throttling controls

Areas which could experience regressions

Loading indicator, window manager (kube shell, container shell, container logs, helm chart logs)

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes