Fix #38: make canvas size responsive to window resizing

[IntelligentSynthesis]

The rdeditor is a well-designed molecular editor built on PySide6, providing a clean and extensible interface for chemical structure editing. To further improve its usability and adaptability, this pull request introduces a responsive canvas size that automatically adjusts to window resizing. In addition, a QComboBox control for adjusting molecular display size has been added, allowing users to conveniently scale molecular structures during editing. These enhancements provide a more flexible and user-friendly experience and resolve issue #38.