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/install-daemon.sh

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+# Install teleportfling as a per-user systemd service (daemon mode).
+#
+# Builds the headless binary, places it in ~/.local/bin, installs the user
+# unit from contrib/teleportfling.service, and creates a starter daemon
+# config. Run without sudo:
+#
+#   ./contrib/install-daemon.sh
+#
+# Then start it with:
+#
+#   systemctl --user enable --now teleportfling
+set -euo pipefail
+
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+BIN_DIR="${HOME}/.local/bin"
+SERVICE_DIR="${HOME}/.config/systemd/user"
+CONFIG_DIR="${HOME}/.config/teleportfling"
+CONFIG="${CONFIG_DIR}/daemon.json"
+
+echo "Building teleportfling..."
+(cd "${HERE}" && go build -o "${BIN_DIR}/teleportfling" ./cmd/teleportfling)
+
+echo "Installing systemd user unit..."
+mkdir -p "${SERVICE_DIR}"
+install -m 0644 "${HERE}/contrib/teleportfling.service" "${SERVICE_DIR}/teleportfling.service"
+
+if [[ ! -f "${CONFIG}" ]]; then
+    echo "Creating default daemon config at ${CONFIG}..."
+    mkdir -p "${CONFIG_DIR}"
+    cat > "${CONFIG}" <<'EOF'
+{
+  "name": "TeleportFling Daemon",
+  "port": 9756,
+  "quality": 80,
+  "fps": 30,
+  "source": "screen",
+  "audio": true,
+  "stream_index": 0,
+  "announce": true
+}
+EOF
+fi
+
+echo "Reloading systemd and enabling the service..."
+systemctl --user daemon-reload
+systemctl --user enable teleportfling
+
+cat <<EOF
+
+Installed. Start the daemon with:
+    systemctl --user start teleportfling
+
+Check status with:
+    systemctl --user status teleportfling
+
+Edit settings in ${CONFIG} (the service passes it via --config).
+EOF