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/views/MainLayout.java

@@ -49,6 +49,8 @@ public class MainLayout extends AppLayout {
                 Fa.FORM.create()));
         nav.addItem(new SideNavItem(getTranslation("nav.table"), TableView.class,
                 Fa.TABLE.create()));
+        nav.addItem(new SideNavItem(getTranslation("nav.gridstack"), GridStackView.class,
+                Fa.GRID.create()));
 
         addToDrawer(nav);
     }