Fix/registry schema validation

[stevenjoezhang]

What changed

This PR extends the existing registry validation so the Tester workflow can catch malformed theme/plugin metadata before maintainers need to review it manually.

It adds validation for:

• Theme YAML schema:
  • required description, link, preview, and non-empty tags
  • unsupported top-level fields
  • optional repository field shape and value types
  • optional author must be a string
• Plugin YAML schema:
  • required description, link, and non-empty tags
  • unsupported top-level fields
  • optional repository field shape and value types
• Case-insensitive registry conflicts:
  • theme names such as Pure vs pure
  • plugin names with case-only differences
  • theme screenshot filenames such as Pure.png vs pure.png

Why

Recent PRs exposed two validation gaps:

• Invalid theme YAML could pass because only screenshot validation was reported.
• Case-only duplicate filenames can merge successfully on GitHub/Linux, but break local checkouts on case-insensitive filesystems such as default macOS.

Test plan

• node --check scripts/validate.js
• Verified invalid fixture-style data reports schema errors locally.

[github-actions]

❌ Theme names conflict by case: Pure, pure.
❌ Theme screenshot filenames conflict by case: Pure.png, pure.png.
❌ Theme thumbnails or registry schema validation failed.