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/form_schema.go

@@ -304,34 +304,47 @@ func decodeFields(raw []byte) ([]FormField, error) {
 }
 
 func (m *fieldMap) UnmarshalYAML(node ast.Node) error {
+	items, err := decodeFieldMapping(node, nil)
+	if err != nil {
+		return err
+	}
+	m.items = items
+	return nil
+}
+
+// decodeFieldMapping compiles a fields mapping in source order. rename, when
+// set, maps a raw field name (such as WinterCMS's pivot[role]) to the name
+// that must be an identifier.
+func decodeFieldMapping(node ast.Node, rename func(string) string) ([]FormField, error) {
 	node = unwrapNode(node)
 	if _, ok := node.(*ast.NullNode); ok || node == nil {
-		m.items = []FormField{}
-		return nil
+		return []FormField{}, nil
 	}
 	mapping, ok := node.(*ast.MappingNode)
 	if !ok {
-		return fmt.Errorf("fields must be a mapping")
+		return nil, fmt.Errorf("fields must be a mapping")
 	}
 	items := make([]FormField, 0, len(mapping.Values))
 	seen := map[string]struct{}{}
 	for _, entry := range mapping.Values {
 		name, err := nodeString(unwrapNode(entry.Key))
+		if err == nil && rename != nil {
+			name = rename(name)
+		}
 		if err != nil || !identifier(name) {
-			return fmt.Errorf("field name %q is not an identifier", nodeText(entry.Key))
+			return nil, fmt.Errorf("field name %q is not an identifier", nodeText(entry.Key))
 		}
 		if _, dup := seen[name]; dup {
-			return fmt.Errorf("duplicate field %s", name)
+			return nil, fmt.Errorf("duplicate field %s", name)
 		}
 		seen[name] = struct{}{}
 		field, err := compileFieldNode(name, unwrapNode(entry.Value))
 		if err != nil {
-			return fmt.Errorf("field %s: %w", name, err)
+			return nil, fmt.Errorf("field %s: %w", name, err)
 		}
 		items = append(items, field)
 	}
-	m.items = items
-	return nil
+	return items, nil
 }
 
 func compileFieldNode(name string, node ast.Node) (FormField, error) {