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/tests/fixtures/lang.json

@@ -309,6 +309,12 @@
     "backend::lang.relation.link": {
       "other": "Dołącz"
     },
+    "backend::lang.relation.load_failed": {
+      "other": "Nie udało się wczytać powiązanych rekordów."
+    },
+    "backend::lang.relation.no_candidates": {
+      "other": "Brak rekordów do dodania."
+    },
     "backend::lang.relation.remove": {
       "other": "Usuń"
     },