fix: add GridStackLayout showcase view (#7)

Adds a /gridstack route demonstrating GridStackLayout: four default
widgets (line/bar/pie chart cards plus a usage hint), buttons to add and
remove widgets at runtime, a reset-layout button and a layout-change
status line. Layout is persisted per browser via the existing
localStorage support. The view gets its own SideNavItem in MainLayout
and full de/en/es translations.

Fixes an infinite recursion in GridStackLayout.add(Component): a
single-argument add(item) resolves to that overload rather than the
varargs one, so its delegation to add(item) called itself. It now
appends through the element API and passes a GridStackItem through
unwrapped.

Adds the first test sources (browserless UI tests) covering the view's
widget count and the add/remove buttons.

Closes #7

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PYAcKoXKPN3rJ7nBroKZkc

src/main/java/com/example/components/GridStackLayout.java

@@ -106,10 +106,17 @@ public class GridStackLayout extends Component implements HasSize, HasStyle {
         }
     }
 
-    /** Convenience overload: wraps a plain component in a default-sized {@link GridStackItem}. */
+    /** Convenience overload: wraps a plain component in a default-sized
+     *  {@link GridStackItem} (an item passed in directly is added as is).
+     *  <p>
+     *  Appends through the element API rather than calling {@code add(item)}:
+     *  a single-argument call resolves to this overload, not the varargs one,
+     *  so delegating would recurse into itself. */
     public GridStackItem add(Component content) {
-        GridStackItem item = new GridStackItem(content);
-        add(item);
+        GridStackItem item = content instanceof GridStackItem existing
+                ? existing
+                : new GridStackItem(content);
+        getElement().appendChild(item.getElement());
         return item;
     }