Add web UI reboot/shutdown controls and system action API

src/webapp.py

@@ -1,5 +1,7 @@
 import json
 import queue
+import subprocess
+import threading
 from typing import Any, Dict
 
 from flask import Flask, Response, jsonify, render_template, request, stream_with_context
@@ -70,6 +72,27 @@ class WebPortal:
             self.network_manager.refresh_state()
             return jsonify({"ok": True, "message": message})
 
+        @self.app.route("/api/system/<action>", methods=["POST"])
+        def system_action(action: str) -> Response:
+            commands = {
+                "reboot": ["systemctl", "reboot"],
+                "shutdown": ["systemctl", "poweroff"],
+            }
+            cmd = commands.get(action)
+            if cmd is None:
+                return jsonify({"ok": False, "message": "Unknown action"}), 400
+
+            self.state.update_status(f"System action requested: {action}", "")
+
+            def _exec() -> None:
+                try:
+                    subprocess.run(cmd, capture_output=True, text=True, timeout=20, check=False)
+                except Exception:
+                    pass
+
+            threading.Thread(target=_exec, daemon=True, name=f"system-{action}").start()
+            return jsonify({"ok": True, "message": f"{action} triggered"})
+
         @self.app.route("/events/serial")
         def serial_events() -> Response:
             @stream_with_context