修复通讯录字母索引条位置错误，与好友列表重叠

.people-index 用了固定 top:13rem，是"新朋友/群组/文件助手/系统消息"
四行快捷入口成为现在这个样式之前遗留的旧值——实测索引条渲染在约173px
处，而按字母分组的好友列表实际从约375px才开始，索引条整整偏高200px，
盖在快捷入口行上而不是贴着好友列表。按真实布局（header+搜索框+四行
入口+分组标题外边距）重新算出27.5rem。

第十二轮最小粒度测试 Ch5-6 发现。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

css/base.css

@@ -9281,7 +9281,14 @@ div.toast.show {
 .people-index {
   position: absolute;
   z-index: 2;
-  top: 13rem;
+  /* Anchored against .people-page (the nearest positioned ancestor), which starts above the
+     header -- 13rem used to land ~200px above the actual friend list (overlapping the fixed
+     4-row entry list: 新朋友/群组/文件助手/系统消息), a leftover value from before that entry
+     list existed in its current form. Real layout above the letter-grouped list is header
+     (4.5rem) + search bar incl. margins (4.4rem) + entry list incl. bottom margin (4 x 4.3rem
+     rows + 0.8rem margin = 18rem) + "好友通讯录" section title's own top margin (0.6rem) =
+     27.5rem -- confirmed live via getBoundingClientRect against the real rendered page. */
+  top: 27.5rem;
   right: 0.35rem;
   display: flex;
   gap: 0.2rem;