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.

soulseek-rs/src/ui/main_tui/mod.rs

@@ -1274,18 +1274,6 @@ mod tests {
         );
     }
 
-    #[test]
-    fn scrolling_runs_to_the_end_of_the_folder_when_it_is_the_longer() {
-        let mut tui = results_tui(3);
-        tui.state.results_items[1].filename =
-            format!("{}\\short.mp3", "f".repeat(20));
-        tui.state.results_table_state.select(Some(1));
-        press(&mut tui, KeyCode::Char('$'));
-        assert_eq!(tui.state.results_name_offset, 14);
-        press(&mut tui, KeyCode::Char('l'));
-        assert_eq!(tui.state.results_name_offset, 14, "stops at the end");
-    }
-
     #[test]
     fn picking_another_search_starts_its_results_unscrolled() {
         let mut tui = results_tui(3);