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_en.properties

@@ -10,16 +10,28 @@ lang.es=Español
 nav.dashboard=Dashboard
 nav.form=Form
 nav.table=Table
+nav.gridstack=Grid
 
 page.dashboard=Dashboard
 page.form=Form
 page.table=Table
+page.gridstack=Grid layout
 
 card.revenueTrend=Revenue Trend
 card.revenueByMonth=Revenue by Month
 card.revenueByRegion=Revenue by Region
 card.registration=Registration
 card.employees=Employees
+card.gridstackHint=How it works
+
+gridstack.hint=Drag cards by their header to move them, resize them from the bottom right corner. The layout is stored in your browser and restored on your next visit.
+gridstack.addWidget=Add widget
+gridstack.removeWidget=Remove last
+gridstack.reset=Reset layout
+gridstack.status=Layout changed – {0} widgets
+gridstack.statusInitial=Layout unchanged
+gridstack.widget=Widget {0}
+gridstack.widgetText=Freely placeable card.
 
 chart.revenueSeries=Revenue 2026
 chart.pointClick=Series {0}, point {1}