Add macOS Catalina 10.15 legacy build lane

[zortos293]

This PR introduces a dedicated macOS 10.15 Catalina build variant that packages the application with Electron 27.3.11 while preserving the same source codebase as the main Electron 41 builds. Catalina artifacts are clearly distinguished in naming to prevent confusion with regular macOS x64 builds.

Packaging Configuration:

• Split electron-builder config from package.json into standalone files: electron-builder.base.json, electron-builder.json, and electron-builder.catalina.json.
• electron-builder.catalina.json pins electronVersion to 27.3.11 and sets mac.minimumSystemVersion to 10.15.0.
• Main builds continue using the Electron 41 line via electron-builder.json.

CI Workflows:

• Added macos-catalina-x64 matrix entry to .github/workflows/auto-build.yml and .github/workflows/release.yml.
• Switched packaging execution from inline npx electron-builder commands to explicit npm scripts for clarity.
• Catalina lane runs the same build output (npm run build) but packages with the legacy config.

Scripts and Naming:

• Added dist:mac:catalina/dist:mac:catalina:signed scripts in opennow-stable/package.json and root package.json.
• Catalina artifacts are named OpenNOW-v${version}-mac-catalina-x64.{dmg,zip} for explicit versioning.

Compatibility:

• No source code changes required; existing session.setPermissionRequestHandler and media access APIs are compatible with Electron 27.

[zortos293]

I'm waiting for an 👍 from an mac os catalina user

[Kief5555]

I'm waiting for an 👍 from an mac os catalina user

Just VM one