feat(12.2-03): add hasMany relation contracts, relation forms and child create

- RelationContract gains Kind (empty is belongsToMany) and ForeignKey, with kind-aware boot checks
- manage.form, view.form and pivot.form compile against the related or pivot model; $/ paths resolve inside the plugin
- view toolbarButtons accept create|update|delete|link|unlink, each the capability of its routes
- POST .../relations/{name}/records creates a child through the manage form; the server sets the hasMany key
- relation schema carries kind, deferrable and the localized forms; 17 new relation message keys in en and pl

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

@@ -2,6 +2,8 @@
   "data": {
     "name": "members",
     "label": "Members",
+    "kind": "belongsToMany",
+    "deferrable": true,
     "view": {
       "list": {
         "columns": [
@@ -72,6 +74,58 @@
       },
       "empty": {
         "other": "This widget has no members."
+      },
+      "create": {
+        "other": "New record"
+      },
+      "createTitle": {
+        "other": "New record"
+      },
+      "updateTitle": {
+        "other": "Edit record"
+      },
+      "previewTitle": {
+        "other": "Record preview"
+      },
+      "created": {
+        "other": "Record created"
+      },
+      "updated": {
+        "other": "Record saved"
+      },
+      "deleteSelected": {
+        "other": "Delete selected"
+      },
+      "deleteConfirm": {
+        "other": "Delete the selected (:count)? This cannot be undone."
+      },
+      "deleteOneConfirm": {
+        "other": "Delete this record? This cannot be undone."
+      },
+      "deleted": {
+        "one": "Deleted :count record",
+        "other": "Deleted :count records"
+      },
+      "pivotTitle": {
+        "other": "Link details"
+      },
+      "pivotSaved": {
+        "other": "Link details saved"
+      },
+      "editPivot": {
+        "other": "Edit link details: :name"
+      },
+      "createSubmit": {
+        "other": "Create record"
+      },
+      "updateSubmit": {
+        "other": "Save record"
+      },
+      "pivotSubmit": {
+        "other": "Save link details"
+      },
+      "linkSubmit": {
+        "other": "Add link"
       }
     }
   },