Since #3971, open_file, save_file, export and screenshot made every path
absolute against the server's working directory, headless as well as
with --connect. Headless, the engine runs in the server's process and
resolves a relative path against the same directory, so the conversion
is only needed for a connected app. It also made a relative path fail
headless when the absolute path is not UTF-8.
The server now makes these paths absolute only for a connected app, as
docs/mcp.md describes. Headless, it sends them as given.