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/app/globals.css

@@ -1,9 +1,26 @@
 @import "tailwindcss";
 
 @theme {
-  --color-primary: #2563eb;
-  --color-primary-dark: #1d4ed8;
-  --radius-card: 12px;
+  --color-background: 0 0% 100%;
+  --color-foreground: 222.2 84% 4.9%;
+  --color-card: 0 0% 100%;
+  --color-card-foreground: 222.2 84% 4.9%;
+  --color-popover: 0 0% 100%;
+  --color-popover-foreground: 222.2 84% 4.9%;
+  --color-primary: 221.2 83.2% 53.3%;
+  --color-primary-foreground: 210 40% 98%;
+  --color-secondary: 210 40% 96.1%;
+  --color-secondary-foreground: 222.2 47.4% 11.2%;
+  --color-muted: 210 40% 96.1%;
+  --color-muted-foreground: 215.4 16.3% 46.9%;
+  --color-accent: 210 40% 96.1%;
+  --color-accent-foreground: 222.2 47.4% 11.2%;
+  --color-destructive: 0 84.2% 60.2%;
+  --color-destructive-foreground: 210 40% 98%;
+  --color-border: 214.3 31.8% 91.4%;
+  --color-input: 214.3 31.8% 91.4%;
+  --color-ring: 221.2 83.2% 53.3%;
+  --radius: 0.5rem;
 }
 
 * {
@@ -16,5 +33,5 @@ body {
   font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
   background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
   min-height: 100vh;
-  color: #1f2937;
+  color: hsl(var(--color-foreground));
 }