feat: add frontend app routing

frontend/src/app/router.tsx

@@ -1,5 +1,30 @@
-import { App } from './App'
+import * as React from 'react'
+
+import { DesignSystemPreview } from '@/features/design-system/DesignSystemPreview'
+
+function getRoute() {
+  return window.location.hash.replace(/^#/, '') || '/tickets'
+}
 
 export function Router() {
-  return <App />
+  const [route, setRoute] = React.useState(getRoute)
+
+  React.useEffect(() => {
+    const handleHashChange = () => setRoute(getRoute())
+    window.addEventListener('hashchange', handleHashChange)
+    return () => window.removeEventListener('hashchange', handleHashChange)
+  }, [])
+
+  if (route === '/design-system') {
+    return <DesignSystemPreview />
+  }
+
+  return (
+    <main className="min-h-screen bg-background p-6 text-foreground">
+      <div className="mx-auto max-w-6xl">
+        <p className="text-sm text-muted-foreground">Route</p>
+        <h1 className="mt-2 text-xl font-semibold">{route}</h1>
+      </div>
+    </main>
+  )
 }