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/useConnections.ts

@@ -0,0 +1,94 @@
+import { ref, watch } from 'vue'
+import type { MqttConnection } from '../types/mqtt'
+import { defaultConnection } from '../types/mqtt'
+
+const STORAGE_KEY = 'mqttx_connections'
+
+function loadConnections(): MqttConnection[] {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY)
+    if (!raw) return []
+    const parsed = JSON.parse(raw) as MqttConnection[]
+    // Ensure each connection has runtime status fields
+    return parsed.map((c) => ({
+      ...c,
+      status: 'disconnected' as const,
+      lastError: '',
+    }))
+  } catch {
+    return []
+  }
+}
+
+function saveConnections(connections: MqttConnection[]) {
+  try {
+    // Strip runtime fields before persisting
+    const toSave = connections.map(({ status: _, lastError: __, ...rest }) => rest)
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(toSave))
+  } catch {
+    // localStorage full or unavailable — silently ignore
+  }
+}
+
+const connections = ref<MqttConnection[]>(loadConnections())
+
+// Auto-persist on changes
+watch(
+  connections,
+  (val) => {
+    saveConnections(val)
+  },
+  { deep: true },
+)
+
+export function useConnections() {
+  function add() {
+    const conn = defaultConnection()
+    connections.value.push(conn)
+    return conn
+  }
+
+  function update(id: string, data: Partial<MqttConnection>) {
+    const idx = connections.value.findIndex((c) => c.id === id)
+    if (idx !== -1) {
+      connections.value[idx] = { ...connections.value[idx], ...data }
+    }
+  }
+
+  function remove(id: string) {
+    const idx = connections.value.findIndex((c) => c.id === id)
+    if (idx !== -1) {
+      connections.value.splice(idx, 1)
+    }
+  }
+
+  function getById(id: string): MqttConnection | undefined {
+    return connections.value.find((c) => c.id === id)
+  }
+
+  function updateStatus(
+    id: string,
+    status: MqttConnection['status'],
+    lastError = '',
+  ) {
+    const conn = getById(id)
+    if (conn) {
+      conn.status = status
+      conn.lastError = lastError
+    }
+  }
+
+  function getConnected(): MqttConnection[] {
+    return connections.value.filter((c) => c.status === 'connected')
+  }
+
+  return {
+    connections,
+    add,
+    update,
+    remove,
+    getById,
+    updateStatus,
+    getConnected,
+  }
+}