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.properties

@@ -10,16 +10,28 @@ lang.es=Español
 nav.dashboard=Dashboard
 nav.form=Formular
 nav.table=Tabelle
+nav.gridstack=Raster
 
 page.dashboard=Dashboard
 page.form=Formular
 page.table=Tabelle
+page.gridstack=Raster-Layout
 
 card.revenueTrend=Umsatz-Entwicklung
 card.revenueByMonth=Umsatz nach Monat
 card.revenueByRegion=Umsatz nach Region
 card.registration=Registrierung
 card.employees=Mitarbeiter
+card.gridstackHint=Bedienung
+
+gridstack.hint=Karten am Titel greifen und verschieben, an der unteren rechten Ecke die Größe ändern. Das Layout wird im Browser gespeichert und beim nächsten Besuch wiederhergestellt.
+gridstack.addWidget=Widget hinzufügen
+gridstack.removeWidget=Letztes entfernen
+gridstack.reset=Layout zurücksetzen
+gridstack.status=Layout geändert – {0} Widgets
+gridstack.statusInitial=Layout unverändert
+gridstack.widget=Widget {0}
+gridstack.widgetText=Frei platzierbare Karte.
 
 chart.revenueSeries=Umsatz 2026
 chart.pointClick=Serie {0}, Punkt {1}