Add missing build/setup instructions to README.md

**File:** README.md
**Section:** Getting Started / Build instructions
**Problem:** The documentation references `gulp build`, but `gulp` is no longer used in the project. Running this command fails.
**Steps to reproduce:**
1. Clone the repo.
2. Run `gulp build` as instructed.
3. Observe the error (command not found).
**Suggested fix:** Replace `gulp build` with `npm run build` in README.md, based on the current build scripts in package.json.
**Environment tested:** Node.js v20.x

Update the build instructions in README.md, using the current scripts in package.json as the reference. Replace the obsolete command with the documented build command, then run npm run build to verify that the instructions work.

Written by the indexing model from the issue text.