I've noticed that /compose route is registered in src/routes/index.js file. But there is no controller for that route as well as a template file. Why is it necessary then?
There is no filter.footer.build footer is rendered separately from the page. And those hooks are only used to add extra data to the page so templates can use them to render new stuff. Easiest is to just create a plugin and copy footer.tpl from your base theme ie persona and add your custom code there.
