Add design and product documentation

DESIGN.md

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+# Design
+
+Status: draft required before major visual implementation.
+
+Define one visual direction, explicit anti-directions, typography, semantic palette, spacing, component language, imagery, motion, responsive behavior, accessibility, and intentional detector exceptions. Reuse shadcn and semantic Tailwind tokens; apply updates only after diff approval.

PRODUCT.md

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+# Product
+
+Status: draft required before major implementation.
+
+Define the audience, job-to-be-done, primary journeys, content model, integrations, non-goals, and measurable acceptance criteria. The AI drafts this file from project specs and applies it only after explicit diff approval.