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/pages/chat/ChatPage.tsx

@@ -0,0 +1,16 @@
+import { useEffect } from 'react'
+import { useParams } from 'react-router-dom'
+import { useChatStore } from '../../store/chatStore'
+import ChatContainer from '../../components/chat/ChatContainer'
+import { api } from '../../api/client'
+
+export default function ChatPage() {
+  const { sessionId } = useParams()
+  const { setSessionId } = useChatStore()
+
+  useEffect(() => {
+    if (sessionId) setSessionId(sessionId)
+  }, [sessionId, setSessionId])
+
+  return <ChatContainer />
+}