Have a more consistent UX between Notebook v7 and v6 in the file manager.

### Problem

For users transitioning from Classic to Lab-based tools while seeking to keep their existing experience (such as we are looking for in berkeley-dsep-infra/datahub#2422), having a consistent UX from Classic to Retro is very important. In our case it's doubly so given the introductory educational context.

To facilitate this, the Retro file manager should match the UX of classic as closely as possible. A few things come to mind from a quick check, but a more thorough audit should be done:

* Offer the checkboxes for multi-file actions. Multi-file operations are possible with modifier keys, but that UX is less discoverable than visible checkboxes, and may pose challenges on mobile devices or other scenarios. It also may present accessibility barriers that the checkbox solves. This should also include the top checkbox with a drop menu.
* The breadcrumbs in Retro should be clickable for navigation, as in Classic.
* The Retro browser should show file size.
* This one is controversial enough (I'm not even 100% sure it's the right thing to do) that I'll open a separate issue for it so it can be discussed on its merits: switch the retro browser to acting on single rather than double clicks...

This list is just a start from a quick review, but overall having as close UX parity as possible in retro would help us tremendously in considering the move of our courses to Retro.

Thanks!!

No files, tests, or entry points are named. Start by auditing the Retro and Classic file managers against the listed differences: multi-file checkboxes, clickable breadcrumbs, and displayed file size. Done means the agreed UX-parity changes are implemented and verified across the relevant file-manager interactions.

Written by the indexing model from the issue text.