Update portal preview UI.

Author: BirjuVachhani

lib/src/transformers/node_transformers/passive_rectangle_transformer.dart

@@ -137,15 +137,7 @@ class PassiveRectangleWidget extends StatelessWidget {
                   Positioned.fill(
                     child: settings.isPreview
                         // Show stripes in preview mode
-                        ? CustomPaint(
-                            painter: StripePainter(
-                              bgColor: const Color(0xFFBA79F4).withOpacity(0.1),
-                              stripeColor: const Color(0xFFBA79F4),
-                              nbOfStripes:
-                                  (node.outerBoxGlobal.size.longestSide / 20)
-                                      .round(),
-                            ),
-                          )
+                        ? PortalPreviewWidget(node: node)
                         : manager.layoutRetrievalBuilder(
                             context,
                             node.innerBoxLocal.size,
@@ -166,6 +158,51 @@ class PassiveRectangleWidget extends StatelessWidget {
   }
 }
 
+class PortalPreviewWidget extends StatelessWidget {
+  final BaseNode node;
+
+  const PortalPreviewWidget({
+    super.key,
+    required this.node,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      clipBehavior: Clip.antiAlias,
+      decoration: BoxDecoration(
+        borderRadius: BorderRadius.circular(4),
+        border: Border.all(
+          color: const Color(0xFFBA79F4),
+          width: 2,
+        ),
+      ),
+      child: CustomPaint(
+        painter: StripePainter(
+          bgColor: const Color(0xFFBA79F4).withOpacity(0.1),
+          stripeColor: const Color(0xFFBA79F4),
+          nbOfStripes: (node.outerBoxGlobal.size.longestSide / 10).round(),
+        ),
+        child: Center(
+          child: Container(
+            padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 6),
+            decoration: BoxDecoration(
+              color: const Color(0xFFBA79F4),
+              borderRadius: BorderRadius.circular(4),
+            ),
+            child: const Text(
+              'Embedded Canvas',
+              style: TextStyle(
+                color: Colors.white,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
 List<Widget> wrapWithPaddingNoScroll(
   BaseNode node,
   List<Widget> children, {