refactor(client): Do not mutate prop directly

Related #6595

src/client/components/timeline.vue

@@ -52,8 +52,8 @@ export default Vue.extend({
 		});
 
 		const prepend = note => {
-			const _note = JSON.parse(JSON.stringify(note));	// deepcopy
-			(this.$refs.tl as any).prepend(_note);
+			Object.freeze(note);
+			(this.$refs.tl as any).prepend(note);
 
 			this.$emit('note');