Release v1.6.6

Remove ARM64 support and simplify to x86_64 only for v1.6.6

• Remove ARM64 build matrix and related workflow complexity
• Delete snap/ directory and snapcraft.yaml file
• Focus on x86_64 Linux builds for reliable CI/CD pipeline
• Fix duplicate build-and-upload job definition in release.yml
• Update CHANGELOG.md for v1.6.6 release
• Ensure consistent and reliable x86_64 Linux desktop builds