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

.vscode/extensions.json

@@ -0,0 +1,7 @@
+{
+  "recommendations": [
+    "Vue.volar",
+    "tauri-apps.tauri-vscode",
+    "rust-lang.rust-analyzer"
+  ]
+}