Fix build: vendored sample helpers, locally-built OpenCV 4.14, working Release config

[Piero-93]

Summary

Makes the project build out-of-the-box (Debug and Release, x64) on a current
Visual Studio 2022 / v143 toolchain, where the original sample_helper_includes
folder and a system-wide OpenCV 4.2 install are not available.

Changes

Build / project

• OpenCV path centralized in a single $(OpenCVInstallDir) user-macro in the
  .vcxproj — change one value to point at your local OpenCV install.
• Updated to OpenCV 4.14 built from source (opencv_world4140d.lib / opencv_world4140.lib),
  replacing the hard-coded opencv_world420d.lib / c:\opencv paths.
• Release configuration now actually builds: it had no OpenCV wiring and was missing
  /std:c++17 (required by std::filesystem). Both are now set.
• Post-build event copies the OpenCV runtime DLL next to the executable
  (it is not provided by the NuGet packages, unlike the Azure Kinect DLLs).
• Removed duplicate NuGet .targets imports (silenced MSB4011 warnings).

Source

• Switched the sample-helper includes from <...> to "..." and vendored
  BodyTrackingHelpers.h / Utilities.h into the repo root, since the upstream
  ..\sample_helper_includes folder is not present.
• Removed dead #include "opencv2/calib3d.hpp" from image_extractor.cpp and
  pointcloud_extractor.cpp (calib3d is never used; avoids needing that module).

Repo hygiene

• .gitignore: ignore the whole /.vs folder and /k4a.
• Removed a redundant, partial local copy of the k4a/ headers (they are provided by
  the Microsoft.Azure.Kinect.Sensor NuGet package).

Docs

• Added PREREQUISITES.md documenting build & runtime requirements, how to point
  the project at a local OpenCV install, and the Debug-vs-Release DLL naming.

Notes for reviewers

• OpenCV is referenced from a local install via $(OpenCVInstallDir) (default
  C:\Temp\opencv\install). Adjust that one macro to your environment — see PREREQUISITES.md.
• Only the core, imgproc, imgcodecs, highgui OpenCV modules are needed.