add optional usage to chatCompleted messages

src/lib/components/chat/Chat.svelte

@@ -836,6 +836,7 @@
 				content: m.content,
 				info: m.info ? m.info : undefined,
 				timestamp: m.timestamp,
+				...(m.usage ? { usage: m.usage } : {}),
 				...(m.sources ? { sources: m.sources } : {})
 			})),
 			model_item: $models.find((m) => m.id === modelId),