Fix that the Segment Statistics were not available in context menus

[philippotto]

... of segment groups

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• use the context menu of a segment group in an annotation and check that the statistics are available for:
  • a new annotation without fallback (should have an index) for a 3d (or 2d) dataset
• it should not be shown for nd annotations or annotations with fallback layer

Issues:

• follow-up to Make ad-hoc meshes compatible with ND segmentations #7394

---

(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment

[hotzenklotz]

Works.