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/FormGrid.vue

@@ -11,12 +11,17 @@ defineProps<{
   errors: Record<string, string[]>
   labels?: RecordMeta['labels']
   source?: ControllerParams | null
+  recordId?: number | null
   idPrefix?: string
 }>()
 const emit = defineEmits<{ update: [name: string, value: unknown] }>()
 
-function spanClass(span: string | undefined): string {
-  switch (span) {
+function spanClass(field: FormFieldSchema): string {
+  // The relation manager always takes the whole row (design screen 5).
+  if (field.type === 'relation-manager') {
+    return 'col-span-full'
+  }
+  switch (field.span) {
     case 'left':
       return 'min-[600px]:col-start-1'
     case 'right':
@@ -35,12 +40,13 @@ function spanClass(span: string | undefined): string {
     <FormField
       v-for="field in fields"
       :key="field.name"
-      :class="spanClass(field.span)"
+      :class="spanClass(field)"
       :field="field"
       :model-value="values[field.name]"
       :errors="errors[field.name]"
       :labels="labels?.[field.name]"
       :source="source"
+      :record-id="recordId"
       :id-prefix="idPrefix"
       @update:model-value="(value: unknown) => emit('update', field.name, value)"
     />