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-tauri/build.rs

@@ -0,0 +1,14 @@
+fn main() {
+    // Workaround for tauri-build issue with cargo's OUT_DIR format change.
+    // The newer cargo version uses `{package}/{hash}/out` instead of `{package}-{hash}/out`,
+    // which breaks the target_dir calculation in tauri-build 2.6.3.
+    // We create the expected `build/build` directory to prevent the fs::read_dir error.
+    let out_dir = std::env::var("OUT_DIR").unwrap();
+    let out_path = std::path::Path::new(&out_dir);
+    if let Some(parent) = out_path.parent().and_then(|p| p.parent()).and_then(|p| p.parent()) {
+        let build_build = parent.join("build");
+        let _ = std::fs::create_dir_all(&build_build);
+    }
+    
+    tauri_build::build()
+}