feat: add itemTitle for structured array item labels

[bobbwal]

Description

Improve structured array labelling in admin forms by introducing explicit singular item labels while preserving backward compatibility for existing array header options.

Changes

• Add itemTitle support for array fields in collection config types.
• Use array field title as the array section heading by default.
• Use itemTitle (when provided) for:
  • array item card label (e.g. Member #1)
  • add button text (e.g. Add Member)
• Keep backward compatibility:
  • retain itemLabel support for array section header
  • keep default add button text as Add item when itemTitle is not provided
• Update docs to document itemTitle usage:
  • packages/core/src/collections/README.md
  • docs/collections-config.md
  • www/src/app/collections/page.mdx
• Rebuild core dist outputs.

Testing

• npm run build:core
• npm test

Unit Tests

• Added/updated unit tests
• All unit tests passing

E2E Tests

• Added/updated E2E tests
• All E2E tests passing

Checklist

• Code follows project conventions
• Tests added/updated and passing
• Type checking passes
• No console errors or warnings
• Documentation updated (if needed)

[lane711]

Hey @bobbwal — thank you! Your itemTitle feature for structured array item labels was cherry-picked and merged as PR #725. It shipped in v2.8.3. Great UX improvement. 🙏

For context: we cherry-pick fork PRs into our own branches to work around GitHub Actions CI limitations with fork secrets. Your authorship is preserved in the git history.