chore: add enqs-headroom user service, set nvim as text/plain handler

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

systemd/user/default.target.wants/enqs-headroom.service

@@ -0,0 +1 @@
+/home/narl/.config/systemd/user/enqs-headroom.service

systemd/user/enqs-headroom.service

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+# Written by `enqs-headroom service install`.
+[Unit]
+Description=Router for <name>.headroom.localhost
+Documentation=https://gitlab.enqs.de/npu/claude-code-enqs/-/wikis/Headroom
+After=network-online.target
+
+[Service]
+Environment=ENQS_HEADROOM_ROUTER_PORT=80
+ExecStart=/home/narl/.local/bin/enqs-headroom serve --port 80
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=default.target