Restore native xterm selection copy

What's Changed

• Copies selected xterm text through the synchronous browser copy event so Ctrl+C works even when the local HTTPS certificate is not yet trusted
• Recognizes Cmd+C as the native macOS copy shortcut while preserving Ctrl+C as SIGINT when no terminal selection exists
• Keeps terminal selections intact after copy and covers Windows, Unix, and macOS shortcut resolution with focused behavior tests