feat(10-04): search, link and unlink related records through the relation manager

- relation-manager registered in the field registry; renders only on an
  existing record, never on create, and is never part of the save body
- RelationManager: relation schema label and comment, debounced search,
  selectable linked list (DataTable relation variant), toolbar buttons in
  declared order, confirmed unlink with plural messages and toasts
- RelationPickerModal: Reka Dialog (aria-modal, focus trap, Esc) over the
  candidates endpoint five per page, selection kept across pages, Dodaj (N)
  POSTs link, focus returns to the opener
- admin OpenAPI documents search, sort, dir, page and per_page on the linked
  and candidate relation routes so the SPA sends them typed
- neutral acme.demo.widgets members fixtures and relation smoke tests

admin/src/components/form/registry.ts

@@ -1,13 +1,15 @@
 // Field renderer registry (D-05). A form field's `type` selects the control
 // component; any type without a renderer gets UnsupportedField, the design's
 // dashed box, so an unknown type never breaks the form. The relation manager
-// arrives with its own component in a later plan. Phase 10.1 turns this seam
-// into the plugin extension point.
+// is registered like any control but holds no form value: it edits its
+// relation through its own endpoints and renders only on an existing record.
+// Phase 10.1 turns this seam into the plugin extension point.
 import type { Component } from 'vue'
 import type { ControllerParams, FormField, RelationOption } from '../../api/types'
 import CheckboxField from './fields/CheckboxField.vue'
 import DropdownField from './fields/DropdownField.vue'
 import NumberField from './fields/NumberField.vue'
+import RelationManager from '../relation/RelationManager.vue'
 import RelationField from './fields/RelationField.vue'
 import SwitchField from './fields/SwitchField.vue'
 import TextField from './fields/TextField.vue'
@@ -27,6 +29,8 @@ export interface FieldControlProps {
   labels?: RelationOption[]
   /** Controller whose option endpoints serve this form; null for settings. */
   source?: ControllerParams | null
+  /** Id of the record being edited; null on create and on settings pages. */
+  recordId?: number | null
 }
 
 const renderers = new Map<string, Component>([
@@ -37,10 +41,17 @@ const renderers = new Map<string, Component>([
   ['switch', SwitchField],
   ['checkbox', CheckboxField],
   ['relation', RelationField],
+  ['relation-manager', RelationManager],
 ])
 
-/** Types whose control shows the label itself (toggle cards). */
-const selfLabelled = new Set<string>(['switch', 'checkbox'])
+/** Types whose control shows the label itself (toggle cards, relation manager). */
+const selfLabelled = new Set<string>(['switch', 'checkbox', 'relation-manager'])
+
+/**
+ * Types that need a saved record and hold no form value: never rendered on
+ * create, never part of the save body (D-05, design screen 5).
+ */
+const recordBound = new Set<string>(['relation-manager'])
 
 export function rendererFor(type: string): Component {
   return renderers.get(type) ?? UnsupportedField
@@ -48,7 +59,12 @@ export function rendererFor(type: string): Component {
 
 /** Whether the SPA can edit values of this field type. */
 export function isRegistered(type: string): boolean {
-  return renderers.has(type)
+  return renderers.has(type) && !recordBound.has(type)
+}
+
+/** Whether a field type renders only on an existing record (relation manager). */
+export function needsRecord(type: string): boolean {
+  return recordBound.has(type)
 }
 
 export function ownsLabel(type: string): boolean {