fix: ensure lowercase rock names

[mrcjkb]

Fixes #287

• Make sure :Rocks install adds entries with lowercase names.
• Show an appropriate error message if :Rocks sync fails to install a rock.
• Validate rocks.toml in health check.

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(installer): some installer bugfix

• [x] Pull request title has the appropriate conventional commit prefix.

If applicable:

• [x] Tested
  • [x] Tests have been added.
  • [x] Tested manually (steps in PR description).
• [ ] Updated documentation.

[vhyrro]

Other than the one review looks solid to me!