Add agent uninstall script and UI reveal

Lets a server be cleanly removed from agent management: uninstall.sh
stops/disables the systemd timer and deletes the unit files, the
installed script, and the credentials env file, without touching the
server's entry or task history in the app. install.sh now prints the
uninstall command on success, and the Servers page has an "Uninstall"
button that reveals the same command (safe to show anytime since it
carries no secret).

agent/linux/install.sh

@@ -69,3 +69,4 @@ echo "Installed. Running an initial report now..."
 "$INSTALL_DIR/schedule-task-manager-agent.sh"
 
 echo "Done. The agent reports every ${INTERVAL_MINUTES} minute(s) via the 'schedule-task-manager-agent.timer' systemd timer."
+echo "To remove it later: curl -fsSL $API_URL/agent/linux/uninstall.sh | bash"