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/resources/vaadin-i18n/translations_es.properties

@@ -10,16 +10,28 @@ lang.es=Español
 nav.dashboard=Panel
 nav.form=Formulario
 nav.table=Tabla
+nav.gridstack=Cuadrícula
 
 page.dashboard=Panel
 page.form=Formulario
 page.table=Tabla
+page.gridstack=Diseño de cuadrícula
 
 card.revenueTrend=Evolución de ingresos
 card.revenueByMonth=Ingresos por mes
 card.revenueByRegion=Ingresos por región
 card.registration=Registro
 card.employees=Empleados
+card.gridstackHint=Cómo funciona
+
+gridstack.hint=Arrastra las tarjetas por su título para moverlas y cambia su tamaño desde la esquina inferior derecha. El diseño se guarda en el navegador y se restaura en la próxima visita.
+gridstack.addWidget=Añadir widget
+gridstack.removeWidget=Quitar el último
+gridstack.reset=Restablecer diseño
+gridstack.status=Diseño modificado – {0} widgets
+gridstack.statusInitial=Diseño sin cambios
+gridstack.widget=Widget {0}
+gridstack.widgetText=Tarjeta de colocación libre.
 
 chart.revenueSeries=Ingresos 2026
 chart.pointClick=Serie {0}, punto {1}