Fall back to plain foot when no foot server is running

[kacperpaczos]

Fixes #228

The Foot entry in the supported-terminals table always launches footclient, but footclient is only a thin client: it needs a foot --server listening on its socket, and exits with code 220 — no terminal window — when there is none. So choosing Foot without a running server made every terminal action silently do nothing.

This mirrors footclient(1)'s socket lookup — $XDG_RUNTIME_DIR/foot-$WAYLAND_DISPLAY.sock, then $XDG_RUNTIME_DIR/foot.sock when $WAYLAND_DISPLAY is unset, then /tmp/foot.sock — and launches plain foot instead when no socket exists (and foot is installed; otherwise the choice is left as it was). With a live server footclient is used exactly as before, so server users keep the fast client. The swap is drop-in: foot and footclient both take the command positionally, and both accept-and-ignore -e for xterm compatibility (foot(1), footclient(1)), which is the separator this table already uses.

In the Flatpak the check asks the host (flatpak-spawn --host test -e), because the sandbox mounts its own runtime dir where a host-side server's socket is not visible — and the terminal itself is spawned on the host anyway.

One assumption worth flagging: the check is "does the default socket path exist", not "can it be connected to". A stale socket left behind by a crashed server still picks footclient, and a server started with a custom --server <path> socket is missed — in that case plain foot still opens a working terminal, just not through the server. I have asked the reporter in #228 whether either case applies to their setup.

Testing

A unit test pins the socket lookup order against footclient(1). End to end, with stub foot/footclient binaries that log their invocations, an isolated settings backend with Foot selected, and XDG_RUNTIME_DIR/WAYLAND_DISPLAY pointed at a scratch directory, clicking Open Terminal gives:

# server socket present
footclient -e distrobox enter fakebox
# socket removed, same session
foot -e distrobox enter fakebox

The flatpak-sandbox branch of the check is the one part I could not exercise end to end here (native install); it degrades to the plain-foot fallback if the host query fails.