feat: fix Docker Windows cross-compile and add color grading

- Fix MXE GPG key import (gpg --dearmor + signed-by)
- Fix MXE package names (opencv4→opencv)
- Use Ubuntu 20.04 base for windows-builder (MXE focal compatibility)
- Install CMake 3.20+ from Kitware PPA for windows-builder
- Add ColorGradingParams.h for color grading pipeline
- Update ColorCorrector, AppConfig, MainWindow, ImageData, CliRunner

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

src/converter/pipeline/ImageData.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "../color/ColorGradingParams.h"
+
 #include <opencv2/core.hpp>
 
 #include <cstdint>
@@ -58,6 +60,7 @@ struct ImageData {
     RawMetadata metadata;                     // Camera/RAW metadata
     FilmType film_type{FilmType::Unknown};    // Detected after Detect stage
     std::optional<cv::Rect> crop_region;      // Set by Crop stage
+    ColorGradingParams color_params{};        // User-defined color grading overrides
 };
 
 } // namespace photoconv