[Docs]: WebUI launch instructions are unclear — users try localhost before starting server

[sorlen008]

The README mentions the WebUI but doesn't make it obvious that you need to run `subsai-webui` first. Tried navigating to `http://localhost:8501\` and got nothing — spent a few minutes thinking it was broken before realizing the server wasn't started.

Suggestion: add a clear "Getting Started" section for the WebUI:

```bash

Start the Web UI

subsai-webui

Then open in your browser:

http://localhost:8501

```

Also worth mentioning:

• The port is 8501 (Streamlit default), not 8080 or 3000 like most people expect
• The command is `subsai-webui`, not `subsai --webui` or `subsai serve`
• On Windows, you may need to run it from a directory that doesn't contain a `subsai` folder (see [Bug]: editable install (pip install -e .) shadows system package, breaks WebUI #188)

Small thing but would save new users some confusion.