feat(system-builder): preview a draft's sheet and its starter (4b3a)

[over2take]

POST /api/systems/preview-sheet answers, for a draft not yet saved, the sheet it is drawn with (effectiveSheet) and the starter sheet CUSTOMIZE copies (sheet.starterSheet). The starter keeps fields for parts that are off, so turning a part back on brings them back on a customized sheet. Main admin only; systemsApi.previewSheet calls it.

Summary

Test plan

• Tested locally
• Tests pass

Pre-merge checklist

Code quality:

• Code follows project style
• No breaking changes (or clearly documented)
• No console errors or warnings

Version & Release:

• Version bumped? If releasing to users, update:
  • frontend/package.json version
  • docker-compose.yml APP_VERSION
  • CHANGELOG.md with release notes
• GitHub Actions will auto-tag Docker images with the new version

Before merging to main:

• All tests passing
• PR reviewed and approved
• Branch is up to date with main
• No merge conflicts

Related issues