Implement full AppImage packaging support including AppRun entry point, .desktop launcher, icon asset, and recursive shared library collection via collect_so_files.sh. The Makefile build target now generates a portable Linux executable by assembling the directory structure, copying the binary to AppImage/usr/bin/r, running the dependency collector, and invoking appimagetool to produce a self-contained AppImage binary. Also update .gitignore to exclude the AppImage build directory and add shell scripts to the ignore list.
