feat(widget): Widget 配置、loader 与右下角悬浮入口

生成可嵌入站点的 loader 与悬浮按钮，支持展开收起和移动端；附独立宿主测试页。

frontend/public/config.js

@@ -0,0 +1,30 @@
+// Runtime configuration - this file gets replaced at container startup
+// Assigned to `self` so the service worker can importScripts() it too;
+// in a page self === window, so window.APP_CONFIG still works.
+self.APP_CONFIG = {
+  // API URLs
+  API_URL: "http://localhost:8000/api/v1",
+  WS_URL: "ws://localhost:8000",
+  WIDGET_URL: "http://localhost:8000",
+  
+  // Firebase Configuration
+  FIREBASE_API_KEY: "",
+  FIREBASE_AUTH_DOMAIN: "",
+  FIREBASE_PROJECT_ID: "",
+  FIREBASE_MESSAGING_SENDER_ID: "680125690355",
+  FIREBASE_APP_ID: "",
+  FIREBASE_STORAGE_BUCKET: "",
+  FIREBASE_MEASUREMENT_ID: "",
+  FIREBASE_VAPID_KEY: "",
+  
+  // Razorpay Checkout
+  RAZORPAY_KEY_ID: "rzp_test_T92OR9gqPBht0N",
+  GTM_ID: "",
+
+  // Google Fonts API
+  GOOGLE_FONTS_API_KEY: "",
+  
+  // Node Environment
+  NODE_ENV: "development",
+  HOST: "0.0.0.0"
+};