fix: make dashboard widgets closable (#14)

Every GridStackItem now renders a close button next to its drag grip,
mirroring the grip's hover affordance. Closing detaches the item
server-side; grid-stack.ts already unregisters widgets via its
MutationObserver, so the layout is persisted without an extra protocol.

Closable is on by default and can be turned off per item with
setClosable(false); GridStackItem.CloseEvent lets views react.

The GridStackView "remove last" toolbar button is dropped — per-widget
close replaces it, so the view no longer tracks a widget stack. Its
counter now only grows, keeping a closed widget's id from being handed
to a new widget (which would inherit the saved position).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ue9ZtWUBQF4SuSHpzZ3zwq

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

@@ -24,15 +24,15 @@ card.registration=Registration
 card.employees=Employees
 card.gridstackHint=How it works
 
-gridstack.hint=Move cards with the grip in their top right corner, resize them from the bottom right corner. The layout is stored in your browser and restored on your next visit.
+gridstack.hint=Move cards with the grip in their top right corner, resize them from the bottom right corner, close them with the X in the top 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.
 gridstack.dragHandle=Move
+gridstack.close=Close
 
 chart.revenueSeries=Revenue 2026
 chart.pointClick=Series {0}, point {1}