Add admin console for managing users; default account is admin

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Dockerfile

@@ -3,7 +3,7 @@ FROM python:3.13-slim
 ENV PYTHONUNBUFFERED=1 DATA_DIR=/data PORT=8080
 RUN useradd --system --uid 10001 --home /app app && mkdir /data && chown app /data
 WORKDIR /app
-COPY server.py index.html ./
+COPY server.py index.html admin.html ./
 USER app
 VOLUME /data
 EXPOSE 8080