/* ==========================================================================
 * 右下角「回到顶部」漂浮按钮（后台 / 前台 / 会员端共用）
 * 由 app/helpers.php 的 back_top_btn() 输出，static/js/back-top.js 控制显隐。
 * 显示规则：默认完全隐藏（看不见也点不到），页面向下滚过 320px 才淡入浮出。
 * 隐藏态用 opacity + visibility + pointer-events，别用 display:none —— 那样做不了淡入。
 * ========================================================================== */
.back-top{position:fixed;right:22px;bottom:34px;z-index:40;
  display:flex;flex-direction:column;align-items:center;justify-content:center;gap:2px;
  width:48px;height:48px;padding:0;border:1px solid #e8ecf2;border-radius:14px;
  background:rgba(255,255,255,.94);color:#4a5568;font-family:inherit;
  font-size:11px;line-height:1.1;box-shadow:0 6px 20px rgba(15,39,67,.14);
  backdrop-filter:blur(6px);cursor:pointer;
  opacity:0;visibility:hidden;pointer-events:none;transform:translateY(10px);
  transition:opacity .22s ease,transform .22s ease,visibility .22s,color .16s,border-color .16s}
.back-top.show{opacity:1;visibility:visible;pointer-events:auto;transform:translateY(0)}
.back-top:hover{border-color:#1f6feb;color:#1f6feb}
.back-top:active{transform:scale(.95)}
.back-top .bt-ic{font-size:16px;line-height:1}
@media (max-width:768px){.back-top{right:14px;bottom:20px;width:44px;height:44px}}
@media (prefers-reduced-motion:reduce){.back-top{transition:none}}