feat: add stage 0 foundation

frontend/src/app/App.tsx

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+import { DesignSystemPreview } from '@/features/design-system/DesignSystemPreview'
+
+export function App() {
+  return <DesignSystemPreview />
+}

frontend/src/app/layout/AppShell.tsx

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+import type * as React from 'react'
+
+import { Sidebar } from './Sidebar'
+import { Topbar } from './Topbar'
+
+type AppShellProps = {
+  children: React.ReactNode
+}
+
+export function AppShell({ children }: AppShellProps) {
+  return (
+    <div className="min-h-screen bg-background text-foreground">
+      <Sidebar />
+      <div className="min-h-screen lg:pl-64">
+        <Topbar />
+        <main>{children}</main>
+      </div>
+    </div>
+  )
+}

frontend/src/app/layout/Sidebar.tsx

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+export function Sidebar() {
+  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>
+    </aside>
+  )
+}

frontend/src/app/layout/Topbar.tsx

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+export function Topbar() {
+  return (
+    <header className="flex h-14 items-center border-b border-border bg-surface px-6">
+      <p className="text-sm text-muted-foreground">工单工作台</p>
+    </header>
+  )
+}

frontend/src/app/router.tsx

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+import { App } from './App'
+
+export function Router() {
+  return <App />
+}