Dockerfile: enable openenv web UI at /web (fixes Space 404)

The README frontmatter has base_path: /web — the HF Space proxies
incoming requests to /web on the container. But openenv's create_app()
only mounts the Gradio web UI at /web when ENABLE_WEB_INTERFACE=true,
which we never set. Result: every Space visitor saw 404 Not Found.

Two changes:
- Set ENABLE_WEB_INTERFACE=true in the container env so create_app()
goes through the create_web_interface_app() branch and mounts the
default OpenEnv Gradio playground at /web.
- Add gradio to the pip install step (web_interface.py imports it).

The /reset, /step, /state, /schema, /ws API endpoints still work as
before; this just adds the UI on top.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -17,11 +17,12 @@ COPY . /app
 # are skipped here because we default to mock. If you need the HF target
 # backend on the Space, override CMD or bake a different image.
 RUN pip install --upgrade pip && \
-    pip install --no-cache-dir "openenv-core[core]>=0.2.2" uvicorn fastapi pydantic && \
+    pip install --no-cache-dir "openenv-core[core]>=0.2.2" uvicorn fastapi pydantic gradio && \
     pip install --no-cache-dir --no-deps -e .
 
 ENV PYTHONUNBUFFERED=1 \
     PROMPT_GOLF_TARGET_BACKEND=mock \
+    ENABLE_WEB_INTERFACE=true \
     HOST=0.0.0.0 \
     PORT=8000