fix(system-builder): center INSTALL's drop box in SYSTEMS.EXE

[over2take]

The drop box gets more room, and the empty status line under every folder no longer reserves a line (or the gap above it), so what sits above it is centered in the panel. The status element stays in place for screen readers.

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