Fix root-group selection in grouped stores

[Copilot]

Root-level variables in Icechunk/Zarr stores could be displayed while the group selector only showed a child group, causing the active group to mismatch the data being shown. This updates the selector so / is surfaced whenever root variables coexist with subgroups, selected by default, and ordered before named groups.

• Root group discovery

  • Detect root-level visible variables and inject / into the group list only when root data exists.
  • Preserve existing behavior for stores that contain only child groups.

• Default selection

  • Initialize the selected group to / when present.
  • Keep the selection stable as datasets change.

• Variable filtering

  • Treat / as the root-group filter and show only root-level variables there.
  • Keep subgroup filtering unchanged for named groups.

Example:

// If both root variables and a child group exist:
["/", "status"]

// If only child groups exist:
["status"]

[eeholmes]

@copilot The changes are now showing / (root) when there are no subgroups

Example
https://earthmover-icechunk-era5.s3.us-east-1.amazonaws.com/era5_surface_aws/spatial
this has only vars at root no subgroup. In this case, the group dropdown should not appear in the UI on left but it does. In fact, ALL datasets are showing / regardless of whether they have any groups.

Task: Fix so that / and group selector dropdown only appears when there are vars AND subgroups at root level.