Add Windows build helper

build_windows.bat

@@ -0,0 +1,42 @@
+@echo off
+setlocal
+
+rem Usage:
+rem   build_windows.bat C:\Qt\6.6.0\mingw_64 C:\opencv\build
+rem   build_windows.bat C:\Qt\6.6.0\msvc2019_64 C:\opencv\build\x64\vc16\lib "Visual Studio 17 2022"
+
+if "%~1"=="" (
+    echo Usage: %~nx0 ^<QtDir^> ^<OpenCV_DIR^> [CMakeGenerator]
+    echo Example MinGW: %~nx0 C:\Qt\6.6.0\mingw_64 C:\opencv\build "MinGW Makefiles"
+    echo Example MSVC : %~nx0 C:\Qt\6.6.0\msvc2019_64 C:\opencv\build\x64\vc16\lib "Visual Studio 17 2022"
+    exit /b 1
+)
+
+if "%~2"=="" (
+    echo Missing OpenCV_DIR.
+    exit /b 1
+)
+
+set QT_DIR=%~1
+set OPENCV_DIR=%~2
+set GENERATOR=%~3
+
+if "%GENERATOR%"=="" (
+    set GENERATOR=MinGW Makefiles
+)
+
+if not exist build-windows (
+    mkdir build-windows
+)
+
+cmake -S . -B build-windows -G "%GENERATOR%" -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH="%QT_DIR%" -DOpenCV_DIR="%OPENCV_DIR%"
+if errorlevel 1 exit /b %errorlevel%
+
+cmake --build build-windows --config Release
+if errorlevel 1 exit /b %errorlevel%
+
+echo.
+echo Build finished.
+echo If the exe cannot start because DLLs are missing, run:
+echo   "%QT_DIR%\bin\windeployqt.exe" build-windows\QtOpenCVImageTool.exe
+echo Then copy OpenCV runtime DLLs to the exe directory.