Add authentication and date picker to Receipt Manager

Add username/password login with session cookies, change-password modal,
and logout. All API endpoints require a valid session. Default credentials
(admin/admin) are auto-created on first run. Also add a date input field
to the receipt modal, defaulting to today but allowing the user to pick
any date.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,12 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY server.py receipts.html ./
+
+EXPOSE 8080
+
+CMD ["python3", "server.py"]