v1.5.1 — 弹窗修复 + 图形验证码 + 用户中心

修复 / Fixes

中文：

English:

Fixed a v1.5.0 bug where all dialogs (login/register, contact, file browser) were squashed against the left edge of the window with broken layout (CSS class-name collision; classes are now isolated and all dialogs center on screen again)
Fixed an account-lock bug where a rejected 3rd-device binding still consumed a device slot, locking out already-bound devices (fixed server-side)

中文：

登录/注册弹窗新增 4 位字母图形验证码（点击图片可刷新），防止批量注册和暴力破解
新增「用户中心」（左侧功能栏入口）：
- 转换额度总览：本月剩余/上限、年度剩余/上限、累计转换次数、年度周期
- 个人资料管理：联系电话、邮箱、公司/单位（用户名不可改）
- 修改密码：校验原密码，改完下次登录生效
- 关于软件：当前版本、最新版本、手动检查更新
界面布局调整：登录/注册入口与登录后的用户信息移至主界面右上方；版本检查统一收纳到用户中心「关于软件」

English:

Added a 4-letter graphical captcha (click to refresh) to the login/register dialog to stop bot registrations and brute-force attempts
Added a "User Center" (left sidebar entry) with: quota overview (monthly/yearly left & limit, total conversions, yearly cycle), profile management (phone/email/company), password change (old password required), and About (current/latest version, manual update check)
UI rearrangement: login/register entry and post-login user info moved to the top-right of the main window; version check consolidated into User Center → About