Impossible to run multithread functions without blocking the UI thread.

Please include the following in your bug report:

**Version of emscripten/emsdk:**
```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.57-git
clang version 19.0.0git
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /opt/homebrew/Cellar/emscripten/3.1.57/libexec/llvm/bin
```

**Failing command line in full:**
No failing command.

**Full link command and output with `-v` appended:**
`-pthread -sPTHREAD_POOL_SIZE=navigator.hardwareConcurrency -O3 -flto -sENVIRONMENT="web, worker" -sFILESYSTEM=0 -sALLOW_MEMORY_GROWTH -sEXPORT_NAME=libxxx -sMODULARIZE`

---

This will generate three files: `libxxx.js`, `libxxx.wasm`, `libxxx.worker.js`.
If I directly use `libxxx.js` in ``, the C function works but it will block the UI thread.
If I try to import `libxxx.js` by using `importScripts("libtinydng.js");` in a custom worker file, it won't work also.
- With `-sEXPORT_NAME=libxxx -sMODULARIZE`, it will cause an error on the `libxxx.worker.js`: `Uncaught TypeError: Failed to execute 'createObjectURL' on 'URL': Overload resolution failed.`
- Without `-sEXPORT_NAME=libxxx -sMODULARIZE`, the custom worker file will be loaded multiple times and multithreading just stop working.