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/README.md

@@ -0,0 +1,40 @@
+# Running TeleportFling as a daemon
+
+`teleportfling` (the headless CLI) can run as a per-user systemd service so
+the stream starts automatically with your graphical session.
+
+## Prerequisites
+
+- The binary must be built (the flake dev shell provides all cgo deps).
+- The screen-capture backend needs the session D-Bus and a PipeWire instance,
+  which is why the unit is a *user* service tied to `graphical-session.target`.
+
+## Install
+
+```sh
+./contrib/install-daemon.sh
+```
+
+This:
+
+1. Builds `cmd/teleportfling` and installs it to `~/.local/bin/`.
+2. Installs `contrib/teleportfling.service` as a user unit.
+3. Creates `~/.config/teleportfling/daemon.json` on first run.
+4. Runs `systemctl --user daemon-reload` and enables the service.
+
+## Manage
+
+```sh
+systemctl --user start teleportfling    # start now
+systemctl --user enable teleportfling   # start at login (done by installer)
+systemctl --user status teleportfling   # check status / logs
+journalctl --user -u teleportfling -f   # follow logs
+systemctl --user stop teleportfling     # stop
+```
+
+## Config
+
+The unit passes `--config ~/.config/teleportfling/daemon.json`. Edit that
+file to change name, port, quality, fps, source, audio or announcement, then
+restart the service. Any flags you add to the `ExecStart=` line override the
+file values.