Fix GPU detection: _utils never imported cv2, so CUDA was always reported absent

get_cuda_device_count() and cuda_build_available() referenced cv2 but _utils.py
never imports it, so they raised NameError (swallowed by except) and always
returned 0/False. GPU was therefore reported unavailable even with a CUDA-enabled
OpenCV — pose(device='gpu') always auto-switched to MediaPipe and the
"built WITHOUT CUDA" message always showed. Import cv2 inside both helpers.

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

Author: alexarje

musicalgestures/_utils.py

@@ -1793,6 +1793,7 @@ def get_cuda_device_count():
              module is unavailable or no devices are detected.
     """
     try:
+        import cv2
         return cv2.cuda.getCudaEnabledDeviceCount()
     except Exception:
         return 0
@@ -1806,6 +1807,7 @@ def cuda_build_available():
         bool: True if OpenCV's build information reports CUDA support, else False.
     """
     try:
+        import cv2
         info = cv2.getBuildInformation()
     except Exception:
         return False