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/App.vue

@@ -0,0 +1,26 @@
+<script setup lang="ts">
+import AppLayout from "./components/AppLayout.vue";
+</script>
+
+<template>
+  <AppLayout />
+</template>
+
+<style>
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+html, body, #app {
+  height: 100%;
+  overflow: hidden;
+}
+
+body {
+  font-family: "Inter", "Helvetica Neue", "PingFang SC", "Microsoft YaHei", sans-serif;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+</style>