v1.5.15 — 离线应急授权（后端故障期间可正常使用）

新增 / Added

中文：

新增离线应急授权能力（企业维护场景）：授权服务器维护、故障或网络中断期间，持有官方签发授权文件的设备可启用「超级管理员」模式——转换不限次数、不消耗账号额度、无需连接授权服务器；删除授权文件即回到普通账号模式，即时生效
授权文件采用 Ed25519 数字签名并绑定设备：防伪造（没有官方私钥无法签发）、防拷贝（复制到其他电脑无效）；客户端只内置公钥，二进制内不存在任何可用凭据
普通用户体验完全不变：注册 / 登录、免费额度、离线兜底转换等逻辑均无改动；授权文件仅随商务支持按需签发

English:

Offline emergency authorization (for maintenance scenarios): while the license server is under maintenance, down, or the network is offline, devices holding an officially issued authorization file can enable "Super Admin" mode — unlimited conversions, no quota consumption, no server connection required; removing the file instantly returns to normal account mode
Authorization files are Ed25519-signed and device-bound: tamper-proof (cannot be issued without the official private key) and non-transferable (invalid when copied to another machine); the client embeds only the public key — no usable credentials exist inside the binary
No change for regular users: registration / login, free quotas, and the offline fallback flow are untouched; authorization files are issued on demand through business channels only