[editor] Fix frame's z-index initialiation

Now it correctly has -1 after initialization.

projects/editor/src/app/services/unit.service.ts

@@ -209,6 +209,7 @@ export class UnitService {
       (newElementProperties as AudioProperties | VideoProperties | ImageProperties).src = mediaSrc;
     }
 
+    // Coordinates are given if an element is dragged directly into a cell
     if (coordinates) {
       newElementProperties.position = {
         ...(section.dynamicPositioning && { gridColumn: coordinates.x }),
@@ -218,6 +219,12 @@ export class UnitService {
       } as PositionProperties;
     }
 
+    // Use z-index -1 for frames
+    newElementProperties.position = {
+      zIndex: elementType === 'frame' ? -1 : 0,
+      ...newElementProperties.position
+    } as PositionProperties;
+
     section.addElement(ElementFactory.createElement({
       type: elementType,
       position: PropertyGroupGenerators.generatePositionProps(newElementProperties.position),