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.

deploy/systemd/mnote-web-prod.service

@@ -0,0 +1,30 @@
+[Unit]
+Description=MNote web production gateway (user)
+Documentation=file:///mnt/Data1T/mnote/scripts/prod-build-start.js
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+# current 软链由 npm run prod:start 指向最新 release 目录
+WorkingDirectory=/mnt/Data1T/mnote/dist/run/mnote-web-prod/current
+# 生产 env 由 prod-build-start 生成（含 .env.all + bind/control-plane 覆盖）
+EnvironmentFile=-/mnt/Data1T/mnote/dist/run/mnote-web-prod/env
+Environment=RUST_LOG=info
+ExecStart=/mnt/Data1T/mnote/dist/run/mnote-web-prod/current/mnote-web
+# 崩溃/被杀后自动拉起
+Restart=always
+RestartSec=3
+# 优雅退出窗口
+TimeoutStopSec=30
+KillMode=mixed
+# 日志落到 journal；release 目录内 mnote-web.log 仍可由 journalctl 旁路查看
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=mnote-web-prod
+
+# 轻量资源上限，避免异常 runaway
+LimitNOFILE=65536
+
+[Install]
+WantedBy=default.target