Exposes the existing remote server mode through a checkbox in Server
Connection settings. When enabled, the server binds to 0.0.0.0 instead
of 127.0.0.1, making it accessible from other devices on the network.
The plumbing already existed (Rust sidecar passes --host 0.0.0.0 when
remote=true, serverStore has mode state, Python backend accepts --host),
but the UI hardcoded startServer(false). This wires it up.
Closes#104
