Fix: disable WASM threads to avoid worker error, set single-thread from start

Rewrite preload.js for @xenova /transformers v2

Fix: set WASM paths to local files before import, use onnxruntime-web node entry

Fix: remove broken env.backends path, use correct v3 API

Fix preload.js: force WASM backend explicitly

Add preload.js - downloads model at build time