feat: add Vercel geo-redirect config for China mainland visitors

Use vercel.json redirects with the x-vercel-ip-country header condition
to route mainland China visitors to the ICP-licenced server.

vercel.json

@@ -0,0 +1,16 @@
+{
+  "redirects": [
+    {
+      "source": "/:path(.*)",
+      "has": [
+        {
+          "type": "header",
+          "key": "x-vercel-ip-country",
+          "value": "CN"
+        }
+      ],
+      "destination": "https://onixbyte.cn/:path*",
+      "permanent": false
+    }
+  ]
+}