fix(llm-proxy): sanitise 503 body (no env-var names in network response)

Oracle round-8 polish caveat: the 503 body still named OPENAI_API_KEY
and ANTHROPIC_API_KEY in the 'hint' field. Buyer-visible in devtools
even though no UI renders it. Replaced with the same buyer-safe copy
the Agent OFF pill uses: 'natural language replies are not enabled in
this environment'.

Image bumped to canvas-llm-proxy:sha-002. Live verified.

llm-proxy/main.py

@@ -136,5 +136,5 @@ async def chat(request: Request) -> JSONResponse:
 
     return JSONResponse(
         status_code=503,
-        content={"error": "no provider configured", "hint": "set OPENAI_API_KEY or ANTHROPIC_API_KEY"},
+        content={"error": "natural language replies are not enabled in this environment"},
     )