Opens a URL in whatever this desktop calls a browser.
$BROWSER first, because someone who set it meant it, then xdg-open, then
the macOS open — the first one that exists on the PATH wins.
The command is run detached from the terminal, with its output on /dev/null:
a browser that decides to log a GTK warning would otherwise write it straight
over the frame the UI has drawn, on a terminal that is in raw mode and will
not scroll it away.
Summary
Functions
Opens url, answering :ok or {:error, message}.
It blocks until the opener returns, which is quick but not instant — call it
from an Atui.Fetch, not from a view callback.