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/widgets.form-schema.json

@@ -115,12 +115,12 @@
         ]
       },
       {
-        "name": "parts",
+        "name": "members",
         "type": "relation-manager",
-        "relation": "parts",
+        "relation": "members",
         "span": "full",
-        "tab": "Parts",
-        "context": "update"
+        "tab": "Members",
+        "comment": "People who may edit this widget"
       }
     ],
     "messages": {