fix: self-heal missing library root on access; accept .zip in admin upload picker

backend/internal/api/libraries.go

@@ -26,6 +26,10 @@ func (a *api) libRoot(c *gin.Context, lib store.Library) (string, bool) {
 		err(c, http.StatusForbidden, "forbidden", "library root outside books dir")
 		return "", false
 	}
+	if e := os.MkdirAll(root, 0o755); e != nil { // 注册库时目录可能尚未落盘,自愈
+		err(c, http.StatusInternalServerError, "internal", "library root")
+		return "", false
+	}
 	return root, true
 }