Prevent adding form steps inside other form steps and fix illegalChildren usage

packages/builder/src/components/design/AppPreview/ComponentSelectionList.svelte

@@ -46,7 +46,7 @@
     <ActionMenu disabled={!item.isCategory}>
       <ActionButton
         icon={item.icon}
-        disabled={isChildAllowed(item, $selectedComponent)}
+        disabled={!item.isCategory && isChildAllowed(item, $selectedComponent)}
         quiet
         size="S"
         slot="control"
@@ -66,6 +66,7 @@
             dataCy={`component-${item.name}`}
             icon={item.icon}
             on:click={() => onItemChosen(item)}
+            disabled={isChildAllowed(item, $selectedComponent)}
           >
             {item.name}
           </MenuItem>

packages/standard-components/manifest.json

@@ -1702,7 +1702,7 @@
     "name": "Form",
     "icon": "Form",
     "hasChildren": true,
-    "illegalChildren": ["section"],
+    "illegalChildren": ["section", "form"],
     "actions": [
       "ValidateForm",
       "ClearForm",
@@ -1764,9 +1764,9 @@
   },
   "formstep": {
     "name": "Form Step",
-    "icon": "Form",
+    "icon": "AssetsAdded",
     "hasChildren": true,
-    "illegalChildren": ["section"],
+    "illegalChildren": ["section", "form", "form step"],
     "styles": ["size"],
     "settings": [
       {