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/FormField.vue

@@ -13,6 +13,7 @@ const props = defineProps<{
   errors?: string[]
   labels?: RelationOption[]
   source?: ControllerParams | null
+  recordId?: number | null
   /** Prefix for element ids, so two forms on a page never collide. */
   idPrefix?: string
 }>()
@@ -46,6 +47,7 @@ const describedBy = computed(() =>
       :described-by="describedBy"
       :labels="labels"
       :source="source"
+      :record-id="recordId"
       @update:model-value="(value: unknown) => emit('update:modelValue', value)"
     />
     <p v-if="showComment" :id="commentId" class="text-[13px] text-muted">{{ field.comment }}</p>