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/test/java/com/example/views/GridStackViewTest.java

@@ -0,0 +1,63 @@
+package com.example.views;
+
+import com.example.Application;
+import com.example.components.GridStackItem;
+import com.example.components.GridStackLayout;
+import com.vaadin.browserless.SpringBrowserlessTest;
+import com.vaadin.browserless.ViewPackages;
+import com.vaadin.flow.component.button.Button;
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+@SpringBootTest(classes = Application.class)
+@ViewPackages(classes = GridStackView.class)
+class GridStackViewTest extends SpringBrowserlessTest {
+
+    @Test
+    void view_rendersGridWithDefaultWidgets() {
+        navigate(GridStackView.class);
+
+        GridStackLayout grid = $view(GridStackLayout.class).first();
+        assertNotNull(grid);
+
+        List<GridStackItem.Position> layout = grid.getLayout();
+        assertEquals(4, layout.size(), "expected the four default widgets");
+        assertTrue(layout.stream().allMatch(p -> p.w() > 0 && p.h() > 0));
+    }
+
+    @Test
+    void addAndRemoveWidget_changesItemCount() {
+        navigate(GridStackView.class);
+
+        GridStackLayout grid = $view(GridStackLayout.class).first();
+        int initial = grid.getLayout().size();
+
+        button("gridstack.addWidget").click();
+        assertEquals(initial + 1, grid.getLayout().size());
+
+        button("gridstack.removeWidget").click();
+        assertEquals(initial, grid.getLayout().size());
+    }
+
+    @Test
+    void removeWidget_onDefaultLayout_keepsStaticWidgets() {
+        navigate(GridStackView.class);
+
+        GridStackLayout grid = $view(GridStackLayout.class).first();
+        button("gridstack.removeWidget").click();
+
+        assertEquals(4, grid.getLayout().size(), "static widgets must not be removable");
+    }
+
+    private Button button(String translationKey) {
+        String caption = getCurrentView().getElement().getComponent()
+                .orElseThrow().getTranslation(translationKey);
+        return $view(Button.class).withText(caption).first();
+    }
+}