Migrate from Google Closure Library and Backbone to Vue.js

[ciembor]

Using Google Closure Library complicates the production build process (requiring JRE + custom build scripts in Python), and Backbone is no longer sufficient to maintain such a large codebase and feels somewhat outdated. After 12 years, I am rewriting the application in Vue.js, which will make it easier in the future to add a backend and enable saving color schemes on the server and sharing them.