Give CASE a brain and a face

Build me a single-file Python web app called case.py using ONLY the Python standard library (http.server) — no pip installs. It should:

• Serve one dark, modern HTML page on http://localhost:4200 with a title

"CASE," a big text input, and a "chat feed" area.

• Have a POST /chat endpoint that calls the LLM API

(use your best available model) with this personality as the system prompt: "You are CASE, a dry, deadpan robot assistant — competent, brief, a little sarcastic. Never use exclamation marks. Keep replies to 2-3 short sentences."

• Read the API key from the ANTHROPIC_API_KEY environment variable.
• When I type and hit enter, show my message and CASE's reply in the feed.

Then run it and tell me how to open it.