Support S-groups in macromolecules mode

[Copilot]

How the feature works? / How did you fix the issue?

S-groups should keep their small-molecules appearance when shown in macromolecules mode, including data S-group field values and nucleotide component S-groups.

• Macromolecules rendering

  • Preserve S-groups when preparing structures for polymer-editor rendering.
  • Keeps brackets, labels, and data field values visible in macromolecules mode.

• Nucleotide component S-group

  • Added rendering support for nucleotide component class labels/icons.
  • Added KET read/write support for nucleotideComponent.

• Coverage

  • Added focused unit coverage for S-group preservation, nucleotide component rendering, and KET serialization/deserialization.

Check list

• unit-tests written
• e2e-tests written
• documentation updated
• PR name follows the pattern #1234 – issue name
• branch name doesn't contain '#'
• PR is linked with the issue
• base branch (master or release/xx) is correct
• task status changed to "Code review"
• reviewers are notified about the pull request