feat(mqtt): implement useMqttWebSocket hook

Was previously a dead stub (TODO: implement). This real implementation
is the foundation for live heartbeats, ping RTT, and the device command
ack flow that later commits build on.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/hooks/useMqttWebSocket.js

@@ -1,2 +1,61 @@
-// TODO: implement
-export function useMqttWebSocket() {}
+// frontend/src/hooks/useMqttWebSocket.js
+// Subscribes to /api/mqtt/ws — broadcasts every inbound MQTT message
+// (status/heartbeat, status/alerts, status/info, logs, data) as it arrives.
+//
+// Usage:
+//   useMqttWebSocket({ onMessage: (msg) => { ... } })
+//   msg shape: { type: 'data'|'status/heartbeat'|..., device_serial, payload, topic }
+
+import { useEffect, useRef, useState } from 'react'
+
+export function useMqttWebSocket({ enabled = true, onMessage } = {}) {
+  const [connected, setConnected] = useState(false)
+  const onMessageRef = useRef(onMessage)
+
+  useEffect(() => {
+    onMessageRef.current = onMessage
+  }, [onMessage])
+
+  useEffect(() => {
+    if (!enabled) return
+
+    let ws = null
+    let reconnectTimer = null
+    let stopped = false
+
+    const connect = () => {
+      const token = localStorage.getItem('access_token')
+      if (!token) return
+
+      const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:'
+      ws = new WebSocket(`${protocol}//${window.location.host}/api/mqtt/ws?token=${token}`)
+
+      ws.onopen = () => setConnected(true)
+      ws.onclose = () => {
+        setConnected(false)
+        if (!stopped) reconnectTimer = setTimeout(connect, 5000)
+      }
+      ws.onerror = () => {}
+      ws.onmessage = (event) => {
+        try {
+          const data = JSON.parse(event.data)
+          onMessageRef.current?.(data)
+        } catch {
+          // ignore invalid JSON
+        }
+      }
+    }
+
+    connect()
+
+    return () => {
+      stopped = true
+      clearTimeout(reconnectTimer)
+      ws?.close()
+    }
+  }, [enabled])
+
+  return { connected }
+}
+
+export default useMqttWebSocket