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-schema.json

@@ -0,0 +1,81 @@
+{
+  "data": {
+    "name": "members",
+    "label": "Members",
+    "view": {
+      "list": {
+        "columns": [
+          {
+            "key": "name",
+            "label": "Name",
+            "searchable": true,
+            "sortable": true
+          },
+          {
+            "key": "email",
+            "label": "E-mail",
+            "searchable": true,
+            "sortable": true
+          }
+        ]
+      },
+      "toolbarButtons": [
+        "link",
+        "unlink"
+      ],
+      "showSearch": true
+    },
+    "manage": {
+      "list": {
+        "columns": [
+          {
+            "key": "name",
+            "label": "Name",
+            "searchable": true,
+            "sortable": true
+          },
+          {
+            "key": "email",
+            "label": "E-mail",
+            "searchable": true,
+            "sortable": true
+          }
+        ]
+      },
+      "toolbarButtons": [],
+      "showSearch": true
+    },
+    "messages": {
+      "link": {
+        "other": "Add member"
+      },
+      "linkHint": {
+        "other": "Pick people who are not members yet. The owner is not listed."
+      },
+      "candidateSearch": {
+        "other": "Search by name or email…"
+      },
+      "linked": {
+        "one": "Added :count member",
+        "other": "Added :count members"
+      },
+      "unlinkSelected": {
+        "other": "Remove selected"
+      },
+      "unlinkConfirm": {
+        "one": "Remove :count member?",
+        "other": "Remove :count members?"
+      },
+      "unlinked": {
+        "one": "Removed :count member",
+        "other": "Removed :count members"
+      },
+      "empty": {
+        "other": "This widget has no members."
+      }
+    }
+  },
+  "meta": {
+    "locale": "en"
+  }
+}