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/components/ui/card.tsx

@@ -0,0 +1,76 @@
+import * as React from "react"
+
+import { cn } from "@/lib/utils"
+
+const Card = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn(
+      "rounded-xl border bg-card text-card-foreground shadow",
+      className
+    )}
+    {...props}
+  />
+))
+Card.displayName = "Card"
+
+const CardHeader = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("flex flex-col space-y-1.5 p-6", className)}
+    {...props}
+  />
+))
+CardHeader.displayName = "CardHeader"
+
+const CardTitle = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("font-semibold leading-none tracking-tight", className)}
+    {...props}
+  />
+))
+CardTitle.displayName = "CardTitle"
+
+const CardDescription = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("text-sm text-muted-foreground", className)}
+    {...props}
+  />
+))
+CardDescription.displayName = "CardDescription"
+
+const CardContent = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div ref={ref} className={cn("p-6 pt-0", className)} {...props} />
+))
+CardContent.displayName = "CardContent"
+
+const CardFooter = React.forwardRef<
+  HTMLDivElement,
+  React.HTMLAttributes<HTMLDivElement>
+>(({ className, ...props }, ref) => (
+  <div
+    ref={ref}
+    className={cn("flex items-center p-6 pt-0", className)}
+    {...props}
+  />
+))
+CardFooter.displayName = "CardFooter"
+
+export { Card, CardHeader, CardFooter, CardTitle, CardDescription, CardContent }