feat: Implement comprehensive mouse event system

Implements multiple mouse event improvements for UI elements:

- Mouse enter/exit events (#140): on_enter, on_exit callbacks and
  hovered property for all UIDrawable types (Frame, Caption, Sprite, Grid)
- Headless click events (#111): Track simulated mouse position for
  automation testing in headless mode
- Mouse move events (#141): on_move callback fires continuously while
  mouse is within element bounds
- Grid cell events (#142): on_cell_enter, on_cell_exit, on_cell_click
  callbacks with cell coordinates (x, y), plus hovered_cell property

Includes comprehensive tests for all new functionality.

Closes #140, closes #111, closes #141, closes #142

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/GameEngine.cpp

@@ -428,6 +428,15 @@ void GameEngine::processEvent(const sf::Event& event)
             actionCode = ActionCode::keycode(event.mouseWheelScroll.wheel, delta );
         }
     }
+    // #140 - Handle mouse movement for hover detection
+    else if (event.type == sf::Event::MouseMoved)
+    {
+        // Cast to PyScene to call do_mouse_hover
+        if (auto* pyscene = dynamic_cast<PyScene*>(currentScene())) {
+            pyscene->do_mouse_hover(event.mouseMove.x, event.mouseMove.y);
+        }
+        return;
+    }
     else
         return;