feat: add manifest.json for PWA and Safari support

frontend/app/manifest.json

@@ -0,0 +1,31 @@
+{
+  "name": "TradingAgentsX",
+  "short_name": "TAX",
+  "description": "由 AI 驅動的多代理 LLM 金融交易框架",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#1E1E2E",
+  "theme_color": "#6B21A8",
+  "icons": [
+    {
+      "src": "/favicon.ico",
+      "sizes": "32x32",
+      "type": "image/x-icon"
+    },
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    },
+    {
+      "src": "/icon.png",
+      "sizes": "1024x1024",
+      "type": "image/png"
+    }
+  ]
+}