Add design and product documentation

DESIGN.md

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+# Design — Northline Clinic
+
+## Direction
+
+Calm planned care rather than spa wellness: warm neutral surfaces, precise typography, clear preparation steps, and trustworthy human imagery.
+
+## System
+
+- Reuse semantic tokens and shadcn controls; do not imitate medical portals with excessive chrome.
+- Use readable measure, strong heading hierarchy, generous tap targets, and explicit form labels.
+- Imagery must feel documentary and respectful, never promise results.
+- Motion is limited to orientation and feedback; reduced-motion behavior is mandatory.
+- Mobile booking keeps privacy and preparation context before submission.
+
+## Intentional patterns
+
+Subtle rules may separate steps and safety notes; accent borders must indicate meaning, not merely style.