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"
     },

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": {

admin/tests/fixtures/widgets.relation-candidates.json

@@ -0,0 +1,81 @@
+{
+  "page1": {
+    "data": [
+      {
+        "id": 11,
+        "name": "Alan Turing",
+        "email": "alan.turing@example.test"
+      },
+      {
+        "id": 12,
+        "name": "Grace Hopper",
+        "email": "grace.hopper@example.test"
+      },
+      {
+        "id": 13,
+        "name": "Emmy Noether",
+        "email": "emmy.noether@example.test"
+      },
+      {
+        "id": 14,
+        "name": "Niels Bohr",
+        "email": "niels.bohr@example.test"
+      },
+      {
+        "id": 15,
+        "name": "Marie Curie",
+        "email": "marie.curie@example.test"
+      }
+    ],
+    "meta": {
+      "page": 1,
+      "per_page": 5,
+      "total": 7,
+      "last_page": 2
+    }
+  },
+  "page2": {
+    "data": [
+      {
+        "id": 16,
+        "name": "Carl Gauss",
+        "email": "carl.gauss@example.test"
+      },
+      {
+        "id": 17,
+        "name": "Rosalind Franklin",
+        "email": "rosalind.franklin@example.test"
+      }
+    ],
+    "meta": {
+      "page": 2,
+      "per_page": 5,
+      "total": 7,
+      "last_page": 2
+    }
+  },
+  "search": {
+    "data": [
+      {
+        "id": 13,
+        "name": "Emmy Noether",
+        "email": "emmy.noether@example.test"
+      }
+    ],
+    "meta": {
+      "page": 1,
+      "per_page": 5,
+      "total": 1,
+      "last_page": 1
+    }
+  },
+  "empty": {
+    "data": [],
+    "meta": {
+      "page": 1,
+      "per_page": 5,
+      "total": 0,
+      "last_page": 0
+    }
+  }
+}

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
+    }
+  }
+}

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"
+  }
+}