feat: initial MQTT v5 debugging tool

- Tauri v2 + Vue 3 + TypeScript + Element Plus
- Connection management (CRUD + enable/disable/test)
- Preset system with {{variable}} placeholders
- Dashboard with live message scrolling + topic tree
- Payload inspector (JSON/Hex/Base64/Plain)
- Protocol-level logging (file + UI dual channel)
- Global variable manager with localStorage persistence
- MQTT v5 property support (user properties, content type, etc.)
- SubAck result tracking with subscribe_tracked API

src/stores/useMessages.ts

@@ -0,0 +1,41 @@
+import { ref, computed } from 'vue'
+import type { MqttMessage } from '../types/mqtt'
+
+const MAX_MESSAGES = 500
+
+const messages = ref<MqttMessage[]>([])
+
+export function useMessages() {
+  function push(msg: MqttMessage) {
+    messages.value.push(msg)
+    if (messages.value.length > MAX_MESSAGES) {
+      messages.value.splice(0, messages.value.length - MAX_MESSAGES)
+    }
+  }
+
+  function clear() {
+    messages.value = []
+  }
+
+  function filterByTopic(topicFilter: string): MqttMessage[] {
+    if (!topicFilter.trim()) return messages.value
+    const filter = topicFilter.trim().toLowerCase()
+    return messages.value.filter((m) => m.topic.toLowerCase().includes(filter))
+  }
+
+  function filterByConnection(connectionId: string): MqttMessage[] {
+    if (!connectionId) return messages.value
+    return messages.value.filter((m) => m.connectionId === connectionId)
+  }
+
+  const messageCount = computed(() => messages.value.length)
+
+  return {
+    messages,
+    push,
+    clear,
+    filterByTopic,
+    filterByConnection,
+    messageCount,
+  }
+}