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:

// 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.