feat(editor): add relation input to reference another collection

[nadaamd]

Adds a relation form input so a string field can reference a document of
another collection, and editors pick it from a searchable list instead of typing
an identifier by hand.

A first step towards #219, which asks for the full feature (arrays, multiple
target collections, MDC props) — see Follow-ups for what is left.
Deliberately not using a closing keyword, so the issue stays open for the rest.

#307 was closed in favour of #219 and asked for the same thing. It proposed an
async options callback on the field; that cannot work, because the options
would have to survive serialisation into the generated JSON Schema that Studio
reads at edit time. Naming the target collection declaratively keeps the schema
static and lets Studio resolve the documents itself.

Why

Cross-collection references are a common content model (post.author →
authors, event.location → locations). Today they render as free-text
fields, so editors copy-paste slugs and a typo is only caught later, by a build
or an integrity check. z.enum() built from the target collection is the usual
workaround, but it renders a closed dropdown and couples the schema to the data.

API

// content.config.ts
author: property(z.string()).editor({
  input: 'relation',
  relation: { collection: 'authors', labelField: 'name' },
})

• collection — name of the referenced collection. Both page and data collections work.
• valueField — field written to the file. Defaults to slug, the document file
  name, which is what a document is usually referenced by. path and stem are
  also available, as is any field of the referenced document.
• labelField — field displayed in the picker. Defaults to name, then title,
  then the stored value.

How it works

InputRelation.vue mirrors InputMedia.vue: a text input plus a searchable
popover. Options come from the existing host bridge — host.document.db.list()
filtered by host.collection.getByFsPath() — so there is no new server route
and nothing external to reach. Search matches both the label and the stored
value, so editors can look up a display name or a slug they already know.

The leading icon reports whether the current value resolves to a document, which
is the main thing a picker can offer over a text field.

Backward compatibility

• The field keeps storing a plain string. File format, content queries and
  existing documents are untouched, and the value stays editable by hand.
• A value matching no document is flagged but never rewritten.
• Fields without input: 'relation' are unaffected; an unknown input already
  falls back to InputText, so an older Studio reading a newer schema degrades
  to a text field.

EditorOptions in @nuxt/content

.editor() forwards its whole argument into $content.editor in the generated
schema, so relation reaches Studio at runtime with no change to
@nuxt/content. Only the types need widening: EditorOptions.input is the
closed union 'media' | 'icon' | 'textarea', so input: 'relation' currently
needs a cast (as in the playground here). That is lifted by the companion PR on
nuxt/content: nuxt/content#3840.

Follow-ups

Deliberately left out to keep this reviewable:

• Arrays of references (z.array(z.string())). The array branch of
  buildFormTreeFromSchema hardcodes type: 'array' and drops the parent's
  editor.input, and InputArray.vue gives string arrays a badge path that
  bypasses InputWrapper. That is the same gap as Add Native Inputs Mapping for array of enum #374 / fix(form): render select dropdowns for array of enum string items #518, so it seemed
  better handled on top of that work than alongside it.
• Multiple target collections (collection: ['blog', 'changelog'] from Collection relationships/references support in form/visual editor #219).
• Documents created in the current unpublished session are not offered until
  they are saved, since options come from the indexed database.
• References in MDC component props.

Test plan

• src/app/test/unit/utils/relation.test.ts — new, 18 cases covering value and
  label resolution, option building and search.
• src/app/test/unit/utils/form.test.ts — 2 cases for the schema → form tree
  mapping, including a relation with no target collection.
• pnpm verify passes: lint clean, nuxt typecheck and
  vue-tsc -p src/app/tsconfig.app.json clean, 413 tests green, pnpm test:types
  clean.
• Exercised in playground/docus, where docs.author now references the
  existing authors data collection.

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