feat: add stage 0 foundation

docs/frontend/design-system.md

@@ -0,0 +1,36 @@
+# Frontend Design System
+
+## Principles
+
+- Build product screens from shared tokens and primitives.
+- Keep operational pages dense, quiet, and optimized for repeated use.
+- Use semantic status colors for ticket state, priority, audit, AI, and system feedback.
+
+## Tokens
+
+Theme tokens live in `frontend/src/styles/globals.css` and are exposed through Tailwind in
+`frontend/tailwind.config.ts`.
+
+Core tokens:
+
+- `background`, `foreground`, `surface`, `border`
+- `muted`, `muted-foreground`
+- `primary`, `success`, `warning`, `danger`, `info`, `ai`
+
+## Components
+
+Base UI primitives:
+
+- `Button`
+- `Badge`
+- `Card`, `CardHeader`, `CardContent`
+
+Product components:
+
+- `AppPageHeader`
+- `AppStatusBadge`
+- `AppPriorityBadge`
+
+Preview page:
+
+- `frontend/src/features/design-system/DesignSystemPreview.tsx`