fix(HEL-69): serve_static 目录 index 兜底，闸门改跳 /m/

审核通过后部署侧小改：目录请求返回 index.html，避免 /m/ 回退桌面页；分流闸门同步改用干净 /m/ 地址。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

app/frontend/index.html

@@ -16,7 +16,7 @@
       const autoMobile = window.matchMedia && window.matchMedia("(max-width: 720px)").matches;
       const mobileUA = /Android|iPhone|iPad|iPod|Mobile|Windows Phone/i.test(navigator.userAgent || "");
       if (!forcedDesktop && !alreadyMobile && (forcedMobile || autoMobile || mobileUA)) {
-        window.location.replace("/m/index.html" + window.location.search + window.location.hash);
+        window.location.replace("/m/" + window.location.search + window.location.hash);
       }
     })();
   </script>