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.relation-linked.json

@@ -0,0 +1,51 @@
+{
+  "all": {
+    "data": [
+      {
+        "id": 21,
+        "name": "Ada Byron",
+        "email": "ada.byron@example.test"
+      },
+      {
+        "id": 22,
+        "name": "Max Planck",
+        "email": "max.planck@example.test"
+      },
+      {
+        "id": 23,
+        "name": "Lise Meitner",
+        "email": "lise.meitner@example.test"
+      }
+    ],
+    "meta": {
+      "page": 1,
+      "per_page": 20,
+      "total": 3,
+      "last_page": 1
+    }
+  },
+  "search": {
+    "data": [
+      {
+        "id": 22,
+        "name": "Max Planck",
+        "email": "max.planck@example.test"
+      }
+    ],
+    "meta": {
+      "page": 1,
+      "per_page": 20,
+      "total": 1,
+      "last_page": 1
+    }
+  },
+  "empty": {
+    "data": [],
+    "meta": {
+      "page": 1,
+      "per_page": 20,
+      "total": 0,
+      "last_page": 0
+    }
+  }
+}