To build the web interface from source, you will need to compile the Go code into a WebAssembly file:
GOOS=js GOARCH=wasm go build -ldflags="-s -w" index.go
This will create a binary file. Compress it with LZMA, encode it in Base64, and paste the final result to L198.
You'll also need to update wasm_exec.js (replace the Go version accordingly) on L197 to glue everything together.