feat: run mnote-web prod under systemd --user with auto-restart

Replace detached spawn with mnote-web-prod.service (Restart=always,
enable + Linger). prod:start installs the unit, writes env/current
symlink, and restarts via systemctl so the gateway survives kills and
logout. Also drop unused std::fs import in web_shell.

rust/crates/mnote-web/src/routes/web_shell.rs

@@ -39,7 +39,6 @@ use control_plane::UpsertNavigationRecentInput;
 use core_protocol::KernelProjectionKind;
 use serde::Deserialize;
 use serde_json::{json, Value};
-use std::fs;
 use std::path::{Component, Path as FsPath, PathBuf};
 
 const HEADER_MNOTE_WEB_OWNER: &str = "x-mnote-web-owner";