Add design and product documentation

DESIGN.md

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+# Design — Monochrome Supply
+
+## Direction
+
+Product-first fashion editorial: monochrome palette, strict grid, large photography, restrained typography, and minimal commerce chrome.
+
+## System
+
+- Reuse semantic tokens; product media owns most visual emphasis.
+- Use sharp alignment and controlled whitespace instead of nested cards.
+- Controls must expose selected, unavailable, hover, focus, and loading states.
+- Motion is subtle and material-aware; disable nonessential transitions for reduced motion.
+- Mobile prioritizes imagery, variant selection, price, delivery, and purchase action.