refactor(tui): trim the folder column change to what it needs

The path splitter is a one-line stdlib call with a single caller, so it
lives beside that caller as a private fn, without its own tests. The pane
tests assert the two columns in one screen fragment each, the end-offset
test keeps only the two cases the existing name tests do not cover, and
the key-handling test that repeated one of them goes. The website mock
splits on the backslash the same way its Info pane already does.

README.md

@@ -98,8 +98,8 @@ underneath. `Tab`/`Shift-Tab` or `1`–`3` move the focus between panes, `z`
 zooms the focused pane to the whole window, and `w` hides it so the others
 widen (its number brings it back). Every list takes `↑`/`↓`, `Home`/`End`,
 `PgUp`/`PgDn` and the vim keys, and `h`/`l` or `←`/`→` scroll a long file name,
-folder or query sideways, `0` and `$` to either end. `PgUp`/`PgDn` scroll a chat log back through its history, and `?`
-shows every key for where you are.
+folder or query sideways, `0` and `$` to either end. `PgUp`/`PgDn` scroll a
+chat log back through its history, and `?` shows every key for where you are.
 
 ### Daemon mode