Add design and product documentation

DESIGN.md

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+# Design
+
+Status: draft required before major visual implementation.
+
+## Required decisions
+
+- Visual direction and anti-directions.
+- Typography, color, spacing, radius, imagery, and icon language.
+- Layout rhythm and responsive priorities.
+- Motion policy, reduced-motion fallback, and performance budget.
+- Accessibility requirements and intentional detector exceptions.
+
+Use semantic Tailwind tokens and existing shadcn primitives. The AI must propose a coherent direction, show the diff, and receive explicit approval before treating it as authoritative.