Add Linux as a supported platform with AppImage, deb, and rpm targets
for both x64 and arm64 architectures.
Changes:
- electron-builder.yml: add linux section with AppImage/deb/rpm targets,
icon, category, and desktop entry configuration
- package.json: add electron:pack:linux script
- electron/main.ts: fix getIconPath() to return .png on Linux instead
of .icns (macOS format); enable loadUserShellEnv() on Linux since GUI
apps launched from desktop also have limited env
- README.md + README_CN.md: update supported platforms section
The README previously stated 'Linux builds are planned. Contributions
welcome.' — this delivers on that.