Skia JPEG re-encode drops original header (72dpi camera default). New
src/utils/jpegDpi.ts rewrites JFIF APP0 density + EXIF X/YResolution
(endian-aware) or inserts minimal JFIF when neither exists; applied via
processAndExportPhoto options.dpi in the camera-capture branch only.
Library-mode exports untouched. Pixels never change - metadata only.
