I15-1: Rethink repo structure a bit

The code has grown a bit organically, it's probably time to tidy it a bit. Specifically:
* `handlers.ts` is just growing quickly, we should slit it by which components it's mocking and put them closer to those components. It's also not clear from the name that it's actually used for mocking
* `queue` should maybe live under `components`?
* `QueueView` is really quite tied to `queue`, should this be pulled in from a `queue` component?

Start by mapping the current locations and imports for `handlers.ts`, `queue`, `QueueView`, and the related components. Decide how mock handlers and queue-specific code should be grouped, then update the repository structure and references so each item lives with its owning component.

Written by the indexing model from the issue text.