Refactor hosts TUI application: remove HelpModal, update help action, and clean up related code

tests/test_main.py

@@ -324,17 +324,12 @@ class TestHostsManagerApp:
             patch("hosts.tui.app.Config", return_value=mock_config),
         ):
             app = HostsManagerApp()
-            app.push_screen = Mock()
+            app.action_show_help_panel = Mock()
 
             app.action_help()
 
-            # Should push the help modal screen
-            app.push_screen.assert_called_once()
-            # Verify the modal is a HelpModal instance
-            from hosts.tui.help_modal import HelpModal
-
-            args = app.push_screen.call_args[0]
-            assert isinstance(args[0], HelpModal)
+            # Should call the built-in help action
+            app.action_show_help_panel.assert_called_once()
 
     def test_action_config(self):
         """Test config action opens modal."""