fix: add a widget registry and "add widget" picker (#21)

"Widget hinzufügen" appended an empty placeholder card, so a widget closed
via its X was gone until a page reload. Widget types now live in a
WidgetRegistry (WidgetDefinition: type id, title key, default size,
content factory); the toolbar button opens a picker dialog over the
registry, and the dashboard builds its initial widgets from it too.

Chart construction moves from DashboardView into the registry factories,
which resolve translation keys through the chart component so they follow
the UI locale. Added widgets get an id of "<type>-<n>" from a
monotonically growing counter, so a closed widget's id is never handed to
a new one.

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

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

@@ -27,8 +27,7 @@ gridstack.addWidget=Add widget
 gridstack.reset=Reset layout
 gridstack.status=Layout changed – {0} widgets
 gridstack.statusInitial=Layout unchanged
-gridstack.widget=Widget {0}
-gridstack.widgetText=Freely placeable card.
+gridstack.pickerTitle=Choose a widget
 gridstack.dragHandle=Move
 gridstack.close=Close