fix: merge GridStackView into DashboardView (#19)

DashboardView and GridStackView rendered the same three charts from the
same hardcoded data, so every chart factory method existed twice. The
dashboard is now the GridStackLayout variant at @Route(""):

- GridStackView deleted, its grid/toolbar/chart factories moved into
  DashboardView, which keeps the point-click notifications the old
  dashboard had.
- Charts size to their widget (100%/100%) instead of a fixed 400px.
- MainLayout has one dashboard entry; nav.gridstack/page.gridstack keys
  dropped from all three bundles.
- Storage key is "dashboard" (was "gridstack-demo"), so a saved layout
  from the old route is not reused.
- GridStackViewTest renamed to DashboardViewTest, plus a test asserting
  the three charts render.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0124BiJikbhbiEsNfJxdWM69

src/main/resources/vaadin-i18n/translations_es.properties

@@ -10,12 +10,10 @@ 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