File list: add a reachable "open" key symmetric with h/left back navigation

**Is your feature request related to a problem? Please describe.**

In the TUI file list, opening the selected document is bound to `enter` only. Going back from a document to the list is bound to `h`, `left`, and `delete` (`ui/ui.go`). For anyone navigating with hjkl this is asymmetric: `h` takes you back, but there is no "forward" key to open — you have to reach for `enter`.

This was raised before in #254 and closed. At that time the objection was that `h`/`l` are used for prev/next page in the file browser, so `l` could not be remapped to "open". That is still true today: the list passes keys to `bubbles/paginator`, whose default keymap binds `h`/`left`/`pgup` to PrevPage and `l`/`right`/`pgdown` to NextPage.

**Describe the solution you'd like**

Add `space` as an "open document" key in the file list, alongside `enter`. `space` is currently unused in the list view, and @meowgorithm already suggested it in #254:

```go
// ui/stash.go, handleDocumentBrowsing
case keyEnter, " ":
```

`space` is easier to reach than `enter` and does not collide with paginator paging.

**Describe alternatives you've considered**

- `l` / `right` to open, but only when the list fits on a single page (no paging to conflict with). This restores the hjkl symmetry in the common case, at the cost of a context-dependent binding.
- A full configurable keymap (#850, PR #917), which is a much larger change.

**Additional context**

glow 3.0.0. Happy to send a PR for the `space` binding if that is acceptable.

Start in ui/stash.go at handleDocumentBrowsing, where the existing enter binding opens the selected document. Check the surrounding file-list handling and ui/ui.go navigation bindings to confirm that space is unused and does not conflict with paginator keys. Done means pressing space opens the selected document just like enter.

Written by the indexing model from the issue text.