fix: undo closing a widget (#40)

Closing a widget was instant and destructive - one misclick on the X
lost its size and position. Close now shows a toast with an undo
button that re-adds the very same widget at its captured gs-x/y/w/h,
keeping its gs-id. Programmatic close() (fromClient=false) is skipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HGRChzremwYCUctf2qMrQz

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

@@ -37,6 +37,8 @@ gridstack.restore=Restaurar
 gridstack.duplicate=Duplicar
 gridstack.remove=Eliminar
 gridstack.export=Exportar como CSV
+gridstack.closed=Widget eliminado
+gridstack.undo=Deshacer
 
 # Encabezado de columna de las exportaciones CSV; la columna de valores lleva
 # el nombre de la serie.