BrowserController render array should return a spinner

Problem/Motivation

As seen in exaggerated form in this video (it's on a throttled connection), the load process is already significantly underway before a "loading" spinner even appears. This is because the spinner doesn't show up until Svelte has initialized. That can take a while sometimes! If \Drupal\project_browser\Controller\BrowserController provides a no-js spinner that is removed once PB is fully initialized, users will have a better sense of what is going on and things will feel less laggy (even if they take the same amount of time).

Right now, even on a fast connection, there's a laggy white screen of nothing before PB kicks in. This should help address that.

Steps to reproduce

Proposed resolution

Remaining tasks

• ✅ File an issue about this project
• ☐ Manual Testing
• ☐ Code Review
• ☐ Accessibility Review
• ☐ Automated tests needed/written?