feat: Integrate shadcn/ui design system

- Install shadcn/ui dependencies (CVA, clsx, tailwind-merge, lucide-react)
- Install Radix UI primitives (slot, label)
- Create utility helper (lib/utils.ts with cn function)
- Update Tailwind config for shadcn/ui theme support
- Add CSS variables for theming in globals.css (Tailwind v4 syntax)
- Add shadcn/ui components: Button, Card, Input, Label
- Update JourneyForm with shadcn/ui components and icons
- Update DataPreview with shadcn/ui components and icons
- Improve UI consistency and accessibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/package.json

@@ -13,14 +13,20 @@
   "license": "ISC",
   "description": "",
   "dependencies": {
+    "@radix-ui/react-label": "^2.1.7",
+    "@radix-ui/react-slot": "^1.2.3",
     "@tailwindcss/postcss": "^4.1.14",
     "@types/node": "^24.7.1",
     "@types/react": "^19.2.2",
     "autoprefixer": "^10.4.21",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "lucide-react": "^0.545.0",
     "next": "^15.5.4",
     "postcss": "^8.5.6",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
+    "tailwind-merge": "^3.3.1",
     "tailwindcss": "^4.1.14",
     "typescript": "^5.9.3"
   }