wip(settings): formula fields JSON-first base and preview

[akira69]

Summary

This PR establishes the JSON-first foundation for Formula Extra Fields.

Intent

• Keep formula authoring centered on the raw JSON Logic editor, preview/sample-values workflow, and save/reopen flow.
• Provide searchable references, operators, and helpers as lookup and copy aids for manual JSON authoring.
• Keep the supported reference surface explicit: current-entity built-ins, current-entity extra.<key> fields, and built-in related-entity references.
• Preserve dependency visibility and delete protection in the Extra Fields table so formula rollout remains reviewable and safe.

Review Focus

• Does the JSON-first workflow make authoring and validation clear?
• Are the reference aids useful without turning this PR into a guided-builder flow?
• Do the table protections and field-link visibility support safe adoption?
• Does the help text consistently reinforce the JSON-first mental model?

Screenshots

Settings view:

Formula editor view:

Validation

Commands:

• uv run ruff check spoolman/derived_fields.py spoolman/extra_fields.py spoolman/formula_references.py spoolman/api/v1/field.py spoolman/api/v1/spool.py spoolman/api/v1/filament.py spoolman/api/v1/vendor.py tests_integration/tests/fields/test_derived.py tests_integration/tests/fields/test_derived_api.py
• ./node_modules/.bin/eslint src/pages/help/index.tsx src/pages/settings/formulaFieldsSettings.tsx src/pages/settings/extraFieldsSettings.tsx
• VITE_APIURL=/api/v1 npm run build

Browser validation on http://localhost:1885:

• Opened Settings -> Extra Fields -> Spools
• Verified the Extra Fields table shows referenced formula keys as code links and disables delete for referenced custom fields
• Created a formula field review_json_test with JSON {"date_only":[{"var":"registered"}]}
• Verified sample values were scaffolded and preview returned derived.review_json_test = 2026-01-01
• Saved the formula field and confirmed it persisted in the table
• Reopened the saved formula field for editing
• Verified the reference-aid area is JSON-first, searchable, and copy-assist oriented
• Loaded /help#formula-fields and checked the help page text matches the JSON-first scope

Test Checklist

• Formula field create flow
• Preview/sample-values flow
• Save and reopen flow
• Extra Fields dependency/delete protection
• Help page copy review
• Frontend lint
• Frontend production build with VITE_APIURL=/api/v1
• Backend/static Ruff check for formula-field files

Follow-up

This PR is intentionally scoped to the JSON-first foundation. Guided insertion and compatibility-driven authoring are handled in stacked PR #13.