feat(spa): custom-widget plugin protocol (#625) + 1.8.0

[MartinCastroAlvarez]

Closes #625. Cross-repo chain complete:

Repo	Released
django-admin-rest-api 1.3.0	PyPI — emits widget: "custom" + widget_class
django-admin-react 1.8.0	this PR — dispatches to consumer-registered widgets

Plugin protocol

A consumer registers a vanilla mount fn for any formfield_overrides widget the SPA doesn't natively render:

<script>
  window.darFieldWidgets = window.darFieldWidgets ?? {};
  window.darFieldWidgets['mypkg.widgets.MarkdownEditor'] = {
    mount(container, props) {
      // Render whatever — vanilla JS, jQuery, mini-React, etc.
      // `props.value`, `props.onChange`, `props.error`, `props.widgetClass`.
      // Return optional cleanup fn called on SPA unmount.
    },
  };
</script>

Vanilla mount fn (not React) so the consumer's widget is framework-agnostic. The SPA wraps it in a thin React effect with a latestProps ref so value/onChange/error reach the widget without re-mounting on every parent render.

Two registration paths

• No-build path: assign to window.darFieldWidgets[<class>] in a regular <script>. Works without an npm publish or consumer-side bundler.
• Module path: import { registerFieldWidget } from '@dar/ui' for consumers shipping their own React module sideloaded into the page. Module-level registrations win on conflict.

Fallback when no registration matches

Default text input + amber note:

Custom widget <class> is not registered; using the default text input.

Operator can still complete the form. Consumers can keep affected models on legacy admin via LEGACY_ADMIN_URL_PREFIX until they wire the widget.

What ships

• custom-widget.ts — registry + registerFieldWidget / lookupFieldWidget.
• CustomWidgetMount.tsx — React adapter for the mount fn (latestProps ref pattern).
• FieldInput.tsx — new branch for widget === 'custom'.
• contract.ts — WidgetHint = … | 'custom'; FieldDescriptor.widget_class?: string.
• README — full "Custom widgets" section with worked example, props table, fallback semantics.
• i18n catalogs (es/pt/fr) — translated the two new fallback strings.

Verification

• pnpm test — 222 / 222 ✓ (up from 216; +6 new)
• poetry run pytest -q — 64 / 64 ✓ on Django 4.2.30
• pnpm -r typecheck ✓
• pnpm lint ✓
• pnpm -w build ✓

Minor bump

1.7.0 → 1.8.0. New user-visible capability. Matches the symmetric 1.3.0 minor on django-admin-rest-api.

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