feat: add frontend app routing

frontend/src/app/App.tsx

@@ -1,5 +1,10 @@
-import { DesignSystemPreview } from '@/features/design-system/DesignSystemPreview'
+import { AppShell } from './layout/AppShell'
+import { Router } from './router'
 
 export function App() {
-  return <DesignSystemPreview />
+  return (
+    <AppShell>
+      <Router />
+    </AppShell>
+  )
 }

frontend/src/app/layout/AppShell.tsx

@@ -13,7 +13,7 @@ export function AppShell({ children }: AppShellProps) {
       <Sidebar />
       <div className="min-h-screen lg:pl-64">
         <Topbar />
-        <main>{children}</main>
+        {children}
       </div>
     </div>
   )

frontend/src/app/layout/Sidebar.tsx

@@ -1,9 +1,27 @@
 export function Sidebar() {
+  const navItems = [
+    { href: '#/tickets', label: 'Tickets' },
+    { href: '#/tickets/new', label: 'Create Ticket' },
+    { href: '#/solutions', label: 'Solutions' },
+    { href: '#/design-system', label: 'Design System' }
+  ]
+
   return (
     <aside className="fixed inset-y-0 left-0 hidden w-64 border-r border-border bg-surface lg:block">
       <div className="border-b border-border px-4 py-3">
         <p className="text-sm font-semibold text-foreground">AI Ticket Platform</p>
       </div>
+      <nav className="flex flex-col gap-1 p-3">
+        {navItems.map((item) => (
+          <a
+            className="rounded-md px-3 py-2 text-sm text-muted-foreground transition-colors hover:bg-muted hover:text-foreground"
+            href={item.href}
+            key={item.href}
+          >
+            {item.label}
+          </a>
+        ))}
+      </nav>
     </aside>
   )
 }

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>
+  )
 }