Fix build and runtime issues in sharpen_rgb_image

[Copilot]

The sharpen_rgb_image project had multiple build-breaking and runtime issues preventing compilation and execution.

Build Issues

• Invalid CMake version: Changed cmake_minimum_required(VERSION 4.1) to VERSION 3.18 (4.1 doesn't exist)
• Missing dependency: Added find_package(Threads REQUIRED) - was linking Threads::Threads without finding the package
• Build artifacts: Added /*/build/ to .gitignore

Runtime Issues

• Hardcoded paths: Replaced absolute paths (/home/muhammad/projects/...) with relative paths (butterfly.bmp, res.bmp)
• Type mismatch: Added explicit cast in single_shared_mem_sharpen_kernel:

  // Before: implicit float->int conversion
  out[g_img_idx].x = clip(channels[0]);
  
  // After: explicit cast
  out[g_img_idx].x = clip(static_cast<int>(channels[0]));

• Uninitialized memory: Set uchar4.w (alpha channel) to 255 during image data copy

Code Quality

• Removed unimplemented apply_sharpen() function (contained only TODO)
• Removed unused double_shared_mem_sharpen_kernel() declaration

Original prompt

On sharpen_rgb_image, what are the issues in that project

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