Fix cropping window stalling on repeated use in the same session

[Copilot]

On Linux, cv2.imshow/cv2.waitKey must run on the main thread. The manual crop UI was invoked via threading.Thread, which worked on the first call but left OpenCV's GUI subsystem in a broken state — causing every subsequent crop call in the same session to stall indefinitely.

Changes

• musicalgestures/_cropvideo.py: Replace threading.Thread with subprocess.run(sys.executable, '_cropping_window.py', ...) in mg_cropvideo_ffmpeg. Each crop invocation now runs in a fresh, isolated process — which is what _cropping_window.py was already designed for (it accepts CLI args and prints w h x y to stdout).
  • Restore the previously commented-out first-frame extraction logic (get_first_frame_as_image, get_box_video_ratio) needed to feed the subprocess.
  • Add subprocess return-code check with stderr in the error message.
  • Add output validation (exactly 4 integers expected) before unpacking.
  • Clean up the temp frame image after subprocess exits.

# Before — threading caused OpenCV GUI state corruption after first use
t = threading.Thread(target=lambda q, arg1: q.put(cropping_window(arg1)), args=(que, filename))
t.start()
t.join()

# After — fresh subprocess per call, no shared GUI state
result = subprocess.run(
    [sys.executable, pyfile, first_frame_as_image, str(scale_ratio), str(scaled_width), str(scaled_height)],
    stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True
)