Add design and product documentation

DESIGN.md

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+# Design — SignalDesk AI
+
+## Direction
+
+Confident B2B SaaS with a precise cobalt accent, strong product evidence, and restrained technical depth. Avoid generic gradient startup styling.
+
+## System
+
+- Reuse semantic Tailwind tokens and existing shadcn primitives.
+- Favor clear hierarchy, compact product screenshots, honest metrics, and one dominant action.
+- Use generous whitespace on marketing pages and denser rhythm only inside dashboard surfaces.
+- Motion explains state changes; respect reduced motion and avoid decorative looping effects.
+- Desktop and mobile must preserve the same content priority and keyboard-visible focus.
+
+## Intentional patterns
+
+Fine separator rules and cobalt side accents are allowed when they communicate grouping or status rather than decoration.