Draft: add Python virtual environment switching function

Introduce a stable and unified Python runtime switching capability for cantor, enabling users to switch with a single click between the system interpreter, virtual environments, and Conda environments.

This MR adds a Python interpreter selector to the frontend settings. At startup, it scans system interpreters as well as venv and Conda environments and validates them; the user’s selection is persisted by writing it to the project configuration. On the backend, it accepts the selected interpreter/environment, determines whether it is a venv or Conda environment, sets PythonHome and ProgramName, and injects PATH, PYTHONPATH, and VIRTUAL_ENV in a platform-specific manner; during switching, it restarts the embedded interpreter and rolls back on failure. This enables one-click switching that ensures run, debug, test, and language services consistently use the same Python runtime.