feat(frontend): add React SPA with chat, auth, and admin pages

- Vite 6 + React 19 + TypeScript + Tailwind 4 + Zustand 5
- Login page with JWT auth
- Chat interface: MessageList, ChatInput, ChatContainer
- WebSocket hook with auto-reconnect and exponential backoff
- Sidebar with new chat button + session navigation
- Admin dashboard placeholder
- 57 modules, 242KB JS, TypeScript zero errors

frontend/web/src/components/chat/MessageList.tsx

@@ -0,0 +1,42 @@
+import type { Message } from '../../types/chat'
+
+export default function MessageList({ messages }: { messages: Message[] }) {
+  if (messages.length === 0) {
+    return (
+      <div className="flex items-center justify-center h-full text-gray-500 text-sm">
+        开始新对话吧~
+      </div>
+    )
+  }
+
+  return (
+    <div className="space-y-4 max-w-3xl mx-auto">
+      {messages.map((msg) => (
+        <MessageBubble key={msg.id} message={msg} />
+      ))}
+    </div>
+  )
+}
+
+function MessageBubble({ message }: { message: Message }) {
+  const isUser = message.role === 'user'
+  const isAction = message.role === 'action'
+
+  if (isAction) {
+    return <div className="text-center text-gray-500 text-sm italic py-1">{message.content}</div>
+  }
+
+  return (
+    <div className={`flex ${isUser ? 'justify-end' : 'justify-start'}`}>
+      <div className={`max-w-[70%] rounded-2xl px-4 py-2.5 text-sm leading-relaxed ${
+        isUser
+          ? 'bg-purple-600 text-white rounded-br-md'
+          : message.role === 'system'
+            ? 'bg-red-900/50 text-red-300 mx-auto text-xs'
+            : 'bg-gray-800 text-gray-100 rounded-bl-md'
+      }`}>
+        {message.content || (message.role === 'assistant' ? <span className="animate-pulse">...</span> : null)}
+      </div>
+    </div>
+  )
+}