Fix Docker build and add labels test data

- Fix Docker build issues:
  * Add missing system dependencies (build-essential, python3-dev, libpq-dev)
  * Fix requirements.txt formatting (separate Flask-Session and openpyxl)
  * Update openpyxl version to 3.1.5 (3.10.0 was invalid)
- Set proper folder permissions for Docker write access (app/ and data/)
- Add comprehensive test data for labels module:
  * 8 sample orders (TEST-ORD-001 through TEST-ORD-008)
  * Mix of printed/unprinted orders for testing
  * Various product types, customers, and delivery dates
  * Ready for QZ Tray printing functionality testing
- Include test data generation scripts for future use
- Application now fully containerized and ready for labels/printing testing

Dockerfile

@@ -11,8 +11,11 @@ ENV PYTHONUNBUFFERED=1 \
 
 # Install system dependencies
 RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential \
+    python3-dev \
     mariadb-client \
     curl \
+    libpq-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # Create application directory