Code fixes for Windows/macOS:
- Only force QT_QPA_PLATFORM=xcb on Linux (it breaks Qt startup on
  Windows/macOS, which have no xcb plugin); respect a pre-set value
- Drop the hardcoded Linux interpreter-path check in check_environment;
  treat dependency version mismatches as warnings, missing packages as fatal
- Portable shebang (/usr/bin/env python3)
- Atomic database download (write to .part, os.replace on success, clean up
  partial files on failure) with a request timeout
- resource_path() helper so the bundled splash image works in frozen builds

Release infrastructure:
- requirements.txt for pip-based install on any OS
- .github/workflows/release.yml: PyInstaller builds for Windows, macOS, and
  Linux published to a GitHub Release on v* tag push
- README: prebuilt download instructions, cross-platform install/run notes