[codex] Add JSON editor and evaluator tooling

[Eis4TY]

Summary

• Add a themed CodeMirror JSON editor and use it for JSON configuration fields plus Dry Run JSON output previews.
• Add evaluator Thinking support, ad-hoc evaluator dry-run preview, and provider health checks.
• Improve dataset column cleanup, result table column visibility behavior, and wide JSON detail dialogs.

Impact

• JSON-heavy configuration and Dry Run output are easier to inspect with syntax highlighting.
• Evaluator setup can be previewed before saving, and provider connectivity can be checked from the dashboard.
• Existing evaluator requests keep the previous wire shape unless Thinking is explicitly enabled.

Validation

• npm run lint
• npm run build