fix: skip offscreen QApplication when launching GUI (issue #88, proper fix)

The previous fix (setting _qt_app=None) was insufficient — PySide6 holds
an internal reference to the QApplication singleton at the C++ level, so
the Python-side deletion didn't actually destroy the object.

Real fix: main() detects the 'gui' subcommand from sys.argv before
dispatching InitPlatformCommand and passes renderer_factory=None, so the
offscreen QApplication is never created in the first place. The gui command
creates its own windowed QApplication in gui/__init__.py:launch().

Also adds sys import that was missing from cli/__init__.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>