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/useSubscriptions.ts

@@ -0,0 +1,78 @@
+import { ref, computed, watch } from 'vue'
+import type { Subscription } from '../types/mqtt'
+
+const STORAGE_KEY = 'mqttx_subscriptions'
+
+function loadSubscriptions(): Subscription[] {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY)
+    if (!raw) return []
+    return JSON.parse(raw) as Subscription[]
+  } catch {
+    return []
+  }
+}
+
+function saveSubscriptions(subs: Subscription[]) {
+  try {
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(subs))
+  } catch {
+    // silently ignore
+  }
+}
+
+const subscriptions = ref<Subscription[]>(loadSubscriptions())
+
+watch(
+  subscriptions,
+  (val) => {
+    saveSubscriptions(val)
+  },
+  { deep: true },
+)
+
+export function useSubscriptions() {
+  function addSubscription(connId: string, topic: string, qos: number = 0) {
+    const existing = subscriptions.value.find(
+      (s) => s.connectionId === connId && s.topic === topic,
+    )
+    if (existing) {
+      existing.qos = qos
+      return false
+    }
+    subscriptions.value.push({
+      id: crypto.randomUUID(),
+      connectionId: connId,
+      topic,
+      qos,
+    })
+    return true
+  }
+
+  function removeSubscription(connId: string, topic: string) {
+    subscriptions.value = subscriptions.value.filter(
+      (s) => !(s.connectionId === connId && s.topic === topic),
+    )
+  }
+
+  function removeAllByConnection(connId: string) {
+    subscriptions.value = subscriptions.value.filter(
+      (s) => s.connectionId !== connId,
+    )
+  }
+
+  function getSubscriptionsByConnection(connId: string): Subscription[] {
+    return subscriptions.value.filter((s) => s.connectionId === connId)
+  }
+
+  const allSubscriptions = computed(() => subscriptions.value)
+
+  return {
+    subscriptions,
+    addSubscription,
+    removeSubscription,
+    removeAllByConnection,
+    getSubscriptionsByConnection,
+    allSubscriptions,
+  }
+}