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

modules/cabana/datepicker_smoke_test.go

@@ -31,6 +31,9 @@ func datepickerFields(extra string) string {
         type: relation-manager
         relation: members
         context: [update]
+    parts:
+        type: relation-manager
+        relation: parts
     lookup:
         label: Lookup
         type: widget