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

phrasebook/backend/lang/en/lang.yaml

@@ -92,6 +92,8 @@ relation:
   search: Search…
   add_count: "Add (:count)"
   candidates_range: ":from–:to of :total"
+  no_candidates: There are no records to add.
+  load_failed: The linked records could not be loaded.
 settings:
   title: Settings
   empty: No settings are available.

phrasebook/backend/lang/pl/lang.yaml

@@ -100,6 +100,8 @@ relation:
   search: Szukaj…
   add_count: "Dodaj (:count)"
   candidates_range: ":from–:to z :total"
+  no_candidates: Brak rekordów do dodania.
+  load_failed: Nie udało się wczytać powiązanych rekordów.
 settings:
   title: Ustawienia
   empty: Brak dostępnych ustawień.