Prevent SIGPIPE from terminating the macOS app

[richardcase]

Problem

UnixSocketConnection.send calls POSIX write(2) without suppressing SIGPIPE. If the daemon closes between the connection check and the write, Darwin can terminate the app instead of returning an error.

Evidence: https://github.com/richardcase/clowder/blob/b5492d5/macos/Sources/ClowderCore/UnixSocketConnection.swift#L86-L100

Acceptance criteria

• Set SO_NOSIGPIPE when creating the socket, or use an equivalent safe write path.
• Preserve clean EPIPE/connection errors for callers.
• Add a test that closes the peer and verifies the process survives.