Ensure features that depend on local cluster continue to work when we don't fetch all clusters

[richard-cox]

Summary

Fixes #15846

Occurred changes and/or fixed issues

• two store getters expected the store to contain all clusters and then find the local cluster from that list
  • isMultiCluster
  • isStandaloneHarvester
• when ssp is enabled for cluster side nav no longer up front fetches all clusters
• this means both getters could not find the local cluster
  • isMultiCluster was always incorrectly false
  • isStandaloneHarvester also feeds into
• both getters were used to determine things in harvester, like the visibility of the side bar in standalone mode (incorrectly shown)
• this is now fixed
  • functional changes are all in shell/store/index
  • when ssp is enabled we manually fetch local cluster
  • when ssp is disabled we already get all clusters
  • in both worlds we now cache the local cluster
  • this is then used straight away to determine harvester single product stuff
  • the cached copy is then used
    • in isMultiCluster, and we also get the count from COUNT resource instead of stored list
    • in isStandaloneHarvester
    • also used in the dynamic content world
• Tidying up
  • fix regression where header.vue wasn't using correct getter to determine if toplevelmenu was shown
  • added clarification on where IF_HAVE.MULTI_CLUSTER was used (not in dashboard, but an extension)
  • snuck in an update to api docs

Technical notes summary

Areas or cases that should be tested

• Standalone Mode - point the harvester extension at the PR and run in standalone mode.
  • setup steps
    • clone https://github.com/harvester/harvester-ui-extension
    • yarn link @rancher/shell
    • RANCHER_ENV=harvester API=<harvester domain> yarn dev
  • validate
    • the side bar (clusters, global products, settings, etc) should not be visible
    • RBAC group should not visible
    • Projects/Namespaces not visible, Namespaces visible
    • Create namespace should not show resource limit and PSA tabs
• Embedded Mode - run the PR against a cluster with a harvester cluster imported to it
  • validate - Virtualization Manager --> click on harv cluster
    • the side bar (clusters, global products, settings, etc) should be visible
    • RBAC --> Cluster Members should be visible ([BUG] When a new Rancher import a Harvester, project/namespace and RBAC menu will disappear harvester/harvester#3701)
    • Projects/Namespaces visible, Namespaces not visible
    • Create namespace should show resource limit and PSA tabs
• with both vai on and off
  • Request to https://updates.rancher.io/rancher/community/updates should contain lkv, lcp, and lnc query params (this request is made once a day, to repeat request again delete the rancher-updates-fetch-next local storage entry)

Areas which could experience regressions

• top level menu placement (given it depends on isMultiCluster)
• users who don't have access to the local cluster should still be able to log in and use the dashboard

Screenshot/Video

Checklist

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• The PR has considered, and if applicable tested with, the three Global Roles Admin, Standard User and User Base

[a110605]

Typo ff -> off.

[richard-cox]

ah, i should have expanded that. ff here means Feature Flag.

I'll sneak an update in another PR

[a110605]

Tested with harvester standalone UI is good. The sidebar doesn't display.