fix(HEL-191): 补回问天解势知识文件兜底并消除 Failed to fetch

宿主机 data 挂载会遮盖镜像内 heaven_knowledge.json；增加不受挂载影响的 seed，
并将缺文件/坏 JSON 转为结构化中文错误，前端展示可读提示而非 Failed to fetch。

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

frontend/shared/api.js

@@ -46,12 +46,29 @@
     }
   }
 
+  function readableRequestError(error) {
+    const message = String(error?.message || "");
+    if (
+      error instanceof TypeError
+      || /failed to fetch|networkerror|load failed|network request failed/i.test(message)
+    ) {
+      return "网络请求失败，服务暂时不可用，请稍后重试。";
+    }
+    return message || "请求失败";
+  }
+
   async function request(url, method = "GET", body = null, options = {}) {
-    const response = await fetch(url, requestOptions(method, body, options.signal));
+    let response;
+    try {
+      response = await fetch(url, requestOptions(method, body, options.signal));
+    } catch (error) {
+      throw new ApiError(readableRequestError(error), 0, null);
+    }
     const payload = await parseJson(response);
     handleUnauthorized(response, url);
     if (!response.ok || payload.error) {
-      throw new ApiError(payload.error || "请求失败", response.status, payload);
+      const message = payload.message || payload.error || "请求失败";
+      throw new ApiError(message, response.status, payload);
     }
     return payload;
   }