Serve the Pi's web UI on port 80 instead of 8080

Just http://<pi-ip> now instead of http://<pi-ip>:8080. Safe since
the systemd service already runs as root (needed for GPIO/SPI access
anyway), so binding the privileged port isn't a new requirement.

Local --mock dev stays on 8080 (README/main.py default unchanged) --
that's normally run without root, and port 80 needs it.

install.sh now does enable + restart instead of enable --now, so
re-running it after a unit-file change (like this one) actually
applies on an already-installed instance, not just a fresh install.

install.sh

@@ -94,7 +94,11 @@ install_service() {
     -e "s#{{PYTHON}}#$VENV_DIR/bin/python3#g" \
     "$APP_DIR/paperwall.service.template" > "$SERVICE_TARGET"
   systemctl daemon-reload
-  systemctl enable --now "$SERVICE_NAME"
+  systemctl enable "$SERVICE_NAME"
+  # restart (not just enable --now) so re-running this after a unit file
+  # change -- e.g. the port -- actually picks it up on an already-running
+  # install, not just on a fresh one.
+  systemctl restart "$SERVICE_NAME"
   echo "  installed and started."
 }
 
@@ -104,7 +108,7 @@ show_status() {
   systemctl status "$SERVICE_NAME" --no-pager -l | head -15 || true
   ip_address=$(hostname -I | awk '{print $1}')
   echo
-  echo "Web UI: http://$ip_address:8080  (or http://$(hostname).local:8080)"
+  echo "Web UI: http://$ip_address  (or http://$(hostname).local)"
   echo "Logs:   journalctl -u $SERVICE_NAME -f"
   echo "Remove: sudo bash uninstall.sh"
   if grep -qE '^\s*dtoverlay=spi0-2cs' "$CONFIG_TXT" 2>/dev/null; then