New browser client

This is a subtask of the next-generation web interface effort (#718).

We should design a new browser interface to go with the new HTTP API. This can start with interface design before the API itself is in place.

It should support:
- [ ] Browsing and querying items (like the current interface) and also albums, ideally with a nice album art display.
- [ ] Playing music in the browser.
- [ ] An interface to the importer. This part seems very hard, so maybe it's best left to the end?

Start with the parent effort in issue #718 and review the current interface and the planned HTTP API. Define the browser interface scope around browsing and querying items and albums, music playback, and the importer; the issue does not identify files or tests, and completion would require settling the design and implementing these capabilities.

Written by the indexing model from the issue text.