Fix organs panel indentation, dead hover box, and nest colon lesion under colon

[aperson30]

Summary

• Rows after the Pancreas subgroup in the Organs panel were indented deeper than the Pancreas header, so colon, duodenum, etc. looked like children of Pancreas. Sibling rows and subgroup headers now share one indent; a subgroup's own children sit one level deeper.
• The empty box that appeared on hover beside each organ did nothing: the spacer next to each row reused the chevron class and inherited its hover background. Replaced with a plain non-interactive spacer (also in Custom Classes).
• colon_lesion is now nested under a Colon subgroup, following the existing Pancreas and Kidneys pattern (header toggles colon, lesion sits below it).

Test plan

• npx tsc -b clean
• npx vitest run 331/331 pass
• Eyeball the Organs panel in the viewer: Digestive System alignment, Colon group with lesion below, no hover box