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/store/chatStore.ts

@@ -0,0 +1,41 @@
+import { create } from 'zustand'
+import type { Message, IotDevice } from '../types/chat'
+
+interface ChatState {
+  messages: Message[]
+  isStreaming: boolean
+  currentSessionId: string | null
+  iotDevices: IotDevice[]
+  inputText: string
+
+  setInput: (text: string) => void
+  setSessionId: (id: string) => void
+  addMessage: (msg: Message) => void
+  appendToLast: (content: string) => void
+  setStreaming: (v: boolean) => void
+  setIotDevices: (d: IotDevice[]) => void
+  clearMessages: () => void
+}
+
+export const useChatStore = create<ChatState>((set, get) => ({
+  messages: [],
+  isStreaming: false,
+  currentSessionId: null,
+  iotDevices: [],
+  inputText: '',
+
+  setInput: (text) => set({ inputText: text }),
+  setSessionId: (id) => set({ currentSessionId: id }),
+  setStreaming: (v) => set({ isStreaming: v }),
+  setIotDevices: (d) => set({ iotDevices: d }),
+
+  addMessage: (msg) => set((s) => ({ messages: [...s.messages, msg] })),
+  appendToLast: (content) => {
+    const msgs = [...get().messages]
+    if (msgs.length > 0) {
+      msgs[msgs.length - 1] = { ...msgs[msgs.length - 1], content: msgs[msgs.length - 1].content + content }
+    }
+    set({ messages: msgs })
+  },
+  clearMessages: () => set({ messages: [] }),
+}))