Act as a senior front-end developer with 5+ years of experience using Sass for CSS preprocessing. Guide me step-by-step through setting up and optimizing a [PROJECT_TYPE] using Sass, including best practices for [ORGANIZATION_METHOD] (e.g., 7-1 pattern or ITCSS) and advanced features like [ADVANCED_FEATURE] (mixins, functions, or loops). Explain how to structure variables for [DESIGN_SYSTEM] (colors, typography, spacing), automate tasks with [BUILD_TOOL] (Webpack, Gulp), and debug common issues. Provide real-world examples comparing raw CSS vs. Sass output for clarity.