refactor: v2 重构 — 模块化拆分 + AI Prompt模板化 + 前端状态管理

P0 - AI交互层:
- ai/prompt.js: 模块化 Prompt 模板
- ai/validator.js: JSON Schema 校验 + normalize
- ai/client.js: DeepSeek API 封装
- 金额计算唯一化，时间格式归一化

P1 - 后端架构:
- routes/: auth/rooms/messages/purchases 独立路由
- middleware/auth.js: 认证授权中间件
- ws/index.js: WebSocket 连接管理
- db.js: 启用 foreign_keys
- server.js: 从588行精简到40行入口

P2 - 前端架构:
- public/css/style.css: CSS 独立
- public/js/store.js: 全局状态 Store
- public/js/{chat,ws,purchases,auth}.js: 功能模块拆分
- index.html: 纯 HTML 结构, v2.6

server/public/js/store.js

@@ -0,0 +1,36 @@
+// 全局状态管理
+const Store = {
+  ws: null,
+  currentRoom: null,
+  rooms: [],
+  pendingUploads: [],
+  messageCache: [],
+  tempBubbleTimer: null,
+  wsReconnectTimer: null,
+  reconnectAttempts: 0,
+  pollTimer: null,
+
+  MAX_RECONNECT_DELAY: 30000,
+
+  getToken() { return localStorage.getItem('token'); },
+  saveAuth(data) {
+    localStorage.setItem('token', data.token);
+    localStorage.setItem('currentUser', data.username);
+    localStorage.setItem('isAdmin', data.isAdmin);
+  },
+  clearAuth() {
+    localStorage.removeItem('token');
+    localStorage.removeItem('currentUser');
+    localStorage.removeItem('isAdmin');
+  },
+  logout() {
+    if (Store.ws) { Store.ws.close(); Store.ws = null; }
+    Store.clearAuth();
+    document.getElementById('login-page').classList.remove('hidden');
+    document.getElementById('main-page').classList.add('hidden');
+    Store.currentRoom = null;
+    Store.rooms = [];
+    document.getElementById('chat-list-container').innerHTML = '';
+    document.getElementById('messages-container').innerHTML = '';
+  },
+};