feat(tui): add Ctrl+←/→ shortcuts to resize files panel

- Update help UI to show “Ctrl+←/→ → resize files panel”.
- Change `set_file_panel_width` to return the clamped width.
- Implement Ctrl+←/→ handling in keyboard input to adjust the files panel width, update status messages, and respect panel collapse state.

crates/owlen-tui/src/ui.rs

@@ -3069,6 +3069,7 @@ fn render_help(frame: &mut Frame<'_>, app: &ChatApp) {
             Line::from("  F1 / ?        → toggle help overlay"),
             Line::from("  :h, :help     → open help from command mode"),
             Line::from("  :files, :explorer → toggle files panel"),
+            Line::from("  Ctrl+←/→      → resize files panel"),
         ],
         1 => vec![
             // Editing
@@ -3181,6 +3182,7 @@ fn render_help(frame: &mut Frame<'_>, app: &ChatApp) {
             Line::from("  :h, :help          → show this help"),
             Line::from("  F1 or ?           → toggle help overlay"),
             Line::from("  :files, :explorer → toggle files panel"),
+            Line::from("  Ctrl+←/→          → resize files panel"),
             Line::from("  :quit              → quit application"),
             Line::from("  Ctrl+C twice       → quit application"),
             Line::from("  :reload            → reload configuration and themes"),