Re-enable ad-hoc mesh computation for volume annotations

[philippotto]

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• create a volume annotation and brush (cell should be at least 4vx wide in each dimension)
• click "compute mesh (ad-hoc)" in mesh tab
• see isosurface in 3D viewport

will look like this:

Issues:

• follow-up for Disable compute-meshfile-button for volume annotations and datasets without segmentation #5648 (see Disable compute-meshfile-button for volume annotations and datasets without segmentation #5648 (comment))

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• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review