feat(repo): one-line macOS installer (curl | bash)

[sepehr-safari]

Summary

Add a one-line macOS installer so users install the latest release with a single
paste — no toolchain, no manual Gatekeeper step:

curl -fsSL https://raw.githubusercontent.com/ocpp-debugkit/studio/main/scripts/install-macos.sh | bash

The script fetches the latest release DMG, verifies its SHA-256 against the
release's published SHA256SUMS, installs OCPP DebugKit Studio.app to
/Applications, clears the download quarantine, and opens it.

Scope

• scripts/install-macos.sh — the installer (fetch → verify → install → open).
• release.yml — publish a SHA256SUMS asset with each release (the installer
  verifies against it; it also lets anyone check a download by hand).
• README — replace the macOS install section with just the one-liner; drop the
  manual download / xattr / notarization prose to reduce confusion. Keep the
  Linux tarball note.
• RELEASING / CHANGELOG.

Notes

• The installer targets the latest release, so it goes live once a release
  carrying SHA256SUMS is published (v0.5.2).
• Verification is integrity against GitHub-over-TLS (both the checksums and the
  DMG come from the release) — not an independent signature; that would need
  notarization / signing keys, which are intentionally out of scope.
• macOS / Apple silicon only (the build is arm64); the script exits with a
  pointer to the tarball otherwise.

Acceptance criteria

• curl … | bash installs and opens the app on a clean machine.
• A checksum mismatch, or a non-macOS / non-arm64 host, fails with a clear
  message.
• shellcheck clean; the README shows only the one-liner for macOS.