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/FieldRenderer.vue

@@ -20,6 +20,7 @@ const control = computed(() => rendererFor(props.field.type))
     :described-by="describedBy"
     :labels="labels"
     :source="source"
+    :record-id="recordId"
     @update:model-value="(value: unknown) => emit('update:modelValue', value)"
   />
 </template>