Fix wrong freezing of Vuex data.

It's the sequence items themselves that benefit from freezing,
not the sequence array itself.

lib/www/client/source/src/store/modules/plan/state.js

@@ -1,5 +1,5 @@
 const state = () => ({
-  sequences: Object.freeze([]),
+  sequences: [],
   remarks: null,
   loading: null,
   timestamp: null,