PostCoffee Style Design System (DESIGN.md, Japanese UI)

Design policy: A clean, bright, pop and friendly D2C specialty coffee EC. A white base with vivid brand colors used effectively as accent colors - Density: EC-style centered on a product grid. Cards (products) arranged at equal intervals, with a practical density that is neither too cramped nor too spacious - Keywords: Clean White / Vivid Green Accent / Playful D2C / Product Grid / Diagnosis-driven - Characteristics: The page background is pure white #ffffff, and extremely light gray #fafafa / #f5f5f7 is used for section dividers. The brand's star is the vivid green #4cc764 (the "New" badge an