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>

backend/http/handler.py

@@ -104,6 +104,13 @@ class HttpTransportMixin:
         except ValueError:
             self.send_error(HTTPStatus.FORBIDDEN)
             return
+        if candidate.is_dir():
+            candidate = (candidate / "index.html").resolve()
+            try:
+                candidate.relative_to(STATIC_DIR.resolve())
+            except ValueError:
+                self.send_error(HTTPStatus.FORBIDDEN)
+                return
         if not candidate.is_file():
             candidate = STATIC_DIR / "index.html"
         try: