Add vim bindings and directory expand/collapse

- Replace arrow keys with j/k for navigation
- Add h/l for collapse/expand directories
- Remove emoji icons, use clean text markers
- Show directories with [-]/[+] expand markers
- Track expanded state per directory path
- Add directory suffix (/) for clarity
- Update help text with vim bindings

src/main.rs

@@ -101,12 +101,18 @@ async fn handle_key_event(state: &mut AppState, key_code: KeyCode) -> Result<()>
         KeyCode::Char('q') => {
             state.should_quit = true;
         }
-        KeyCode::Up => {
+        KeyCode::Char('k') => {
             state.move_selection_up();
         }
-        KeyCode::Down => {
+        KeyCode::Char('j') => {
             state.move_selection_down();
         }
+        KeyCode::Char('h') => {
+            state.collapse_selected();
+        }
+        KeyCode::Char('l') => {
+            state.expand_selected();
+        }
         KeyCode::Enter => {
             if let Some(item) = state.get_selected_item() {
                 if !item.node.is_dir {