feat: add systemd user service for daemon mode

- contrib/teleportfling.service: per-user unit running the headless CLI
  with its own config (~/.config/teleportfling/daemon.json)
- contrib/install-daemon.sh: builds the binary, installs the unit and a
  starter daemon config
- contrib/README.md: install and management instructions
- todo.md: mark --config and daemon items complete

contrib/teleportfling.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=TeleportFling screen and audio stream sender
+Documentation=https://github.com/fzwoch/obs-teleport
+After=graphical-session.target pipewire.service
+PartOf=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=%h/.local/bin/teleportfling --config %h/.config/teleportfling/daemon.json
+Restart=on-failure
+RestartSec=2
+# The capture backend needs the session bus and a PipeWire instance.
+Environment=XDG_RUNTIME_DIR=%t
+
+[Install]
+WantedBy=default.target