Fix reference to BlockGrid layoutStylesheet and add mock

src/Umbraco.Web.UI.Client/src/mocks/data/data-type/data-type.data.ts

@@ -702,6 +702,10 @@ export const data: Array<UmbMockDataTypeModel> = [
 				alias: 'blockGroups',
 				value: [{ key: 'demo-block-group-id', name: 'Demo Blocks' }],
 			},
+      {
+        alias: 'layoutStylesheet',
+        value: '/wwwroot/css/umbraco-blockgridlayout.css'
+      },
 			{
 				alias: 'blocks',
 				value: [

src/Umbraco.Web.UI.Client/src/packages/block/block-grid/context/block-grid-manager.context.ts

@@ -29,7 +29,7 @@ export class UmbBlockGridManagerContext<
 
 		if (layoutStylesheet) {
 			// Cause we await initAppUrl in setting the _editorConfiguration, we can trust the appUrl begin here.
-			return this.#appUrl! + removeInitialSlashFromPath(transformServerPathToClientPath(layoutStylesheet));
+			return this.#appUrl! + transformServerPathToClientPath(layoutStylesheet);
 		}
 		return undefined;
 	});

src/Umbraco.Web.UI.Client/src/packages/block/block-type/components/block-type-card/block-type-card.element.ts

@@ -28,7 +28,7 @@ export class UmbBlockTypeCardElement extends UmbLitElement {
 		value = transformServerPathToClientPath(value);
 		if (value) {
 			this.#init.then(() => {
-				this._iconFile = this.#appUrl + removeInitialSlashFromPath(value);
+				this._iconFile = this.#appUrl + value;
 			});
 		} else {
 			this._iconFile = undefined;