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

todo.md

@@ -10,9 +10,12 @@ Backlog of deferred / planned work, tracked outside of milestone milestones.
 - [ ] **Surface runtime errors in the GUI** — capture/encode failures currently
       only hit the log. Show them in the status label (e.g. a denied
       screen-share dialog) so problems are visible without reading logs.
-- [ ] **`--config` flag** — allow the CLI and GUI to point at a custom config
-      path (`teleportfling --config /path/config.json`). Useful for a future
-      daemon or service and for running multiple profiles side by side.
+- [x] **`--config` flag** — the CLI and GUI accept a custom config path
+      (`teleportfling --config /path/config.json`); explicit CLI flags
+      override file values. Used by the systemd daemon unit in contrib/.
+- [x] **Run as a daemon** — `contrib/install-daemon.sh` installs a per-user
+      systemd unit (`contrib/teleportfling.service`) that starts the headless
+      CLI with its own config at `~/.config/teleportfling/daemon.json`.
 
 ## Ideas (not yet scoped)