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/ChatContainer.tsx

@@ -0,0 +1,47 @@
+import { useEffect, useRef } from 'react'
+import { useChatStore } from '../../store/chatStore'
+import { useWebSocket } from '../../hooks/useWebSocket'
+import MessageList from './MessageList'
+import ChatInput from './ChatInput'
+
+export default function ChatContainer() {
+  const { send } = useWebSocket()
+  const { messages, isStreaming, currentSessionId, inputText, setInput, addMessage, setStreaming } = useChatStore()
+  const bottomRef = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    bottomRef.current?.scrollIntoView({ behavior: 'smooth' })
+  }, [messages])
+
+  const handleSend = () => {
+    if (!inputText.trim() || !currentSessionId) return
+
+    const userMsg = {
+      id: crypto.randomUUID(), session_id: currentSessionId,
+      role: 'user' as const, content: inputText, created_at: new Date().toISOString(),
+    }
+    addMessage(userMsg)
+    setStreaming(true)
+
+    // Add placeholder for assistant response
+    addMessage({ id: crypto.randomUUID(), session_id: currentSessionId, role: 'assistant', content: '', created_at: new Date().toISOString() })
+
+    send({ type: 'message', content: inputText, session_id: currentSessionId, timestamp: Date.now() })
+    setInput('')
+  }
+
+  return (
+    <div className="flex flex-col h-full">
+      <div className="flex-1 overflow-auto px-4 py-4">
+        <MessageList messages={messages} />
+        <div ref={bottomRef} />
+      </div>
+      <ChatInput
+        value={inputText}
+        onChange={setInput}
+        onSend={handleSend}
+        disabled={isStreaming}
+      />
+    </div>
+  )
+}

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

@@ -0,0 +1,38 @@
+interface ChatInputProps {
+  value: string
+  onChange: (v: string) => void
+  onSend: () => void
+  disabled?: boolean
+}
+
+export default function ChatInput({ value, onChange, onSend, disabled }: ChatInputProps) {
+  const handleKeyDown = (e: React.KeyboardEvent) => {
+    if (e.key === 'Enter' && !e.shiftKey) {
+      e.preventDefault()
+      onSend()
+    }
+  }
+
+  return (
+    <div className="border-t border-gray-800 p-3 shrink-0">
+      <div className="max-w-3xl mx-auto flex gap-2">
+        <input
+          type="text"
+          value={value}
+          onChange={(e) => onChange(e.target.value)}
+          onKeyDown={handleKeyDown}
+          disabled={disabled}
+          placeholder={disabled ? '思考中...' : '输入消息...'}
+          className="flex-1 bg-gray-800 rounded-lg px-4 py-2.5 text-sm outline-none focus:ring-2 focus:ring-purple-500 disabled:opacity-50"
+        />
+        <button
+          onClick={onSend}
+          disabled={disabled || !value.trim()}
+          className="bg-purple-600 hover:bg-purple-500 disabled:bg-gray-700 rounded-lg px-5 py-2.5 text-sm font-medium transition-colors disabled:cursor-not-allowed"
+        >
+          发送
+        </button>
+      </div>
+    </div>
+  )
+}

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>
+  )
+}