feat(tui): display key hints in status bar and bind “?” to open help

- Add placeholder span showing shortcuts (i:Insert, m:Model, ?:Help, : Command) in the UI footer.
- Insert help section describing Enter key behavior in normal and insert modes.
- Extend F1 help shortcut to also trigger on “?” key (with no or Shift modifier).

crates/owlen-tui/src/chat_app.rs

@@ -1290,7 +1290,12 @@ impl ChatApp {
                     }
                 }
 
-                if matches!(key.code, KeyCode::F(1)) {
+                if matches!(key.code, KeyCode::F(1))
+                    || matches!(
+                        (key.code, key.modifiers),
+                        (KeyCode::Char('?'), KeyModifiers::NONE | KeyModifiers::SHIFT)
+                    )
+                {
                     self.mode = InputMode::Help;
                     self.status = "Help".to_string();
                     return Ok(AppState::Running);