群昵称改用专门的接口读取，修好读不回已保存值的问题

上一个commit修好了"群资料"页把群名称当成个人群昵称显示的问题，但实机验证发现改完仍然显示"未设置"。
抓包确认根因：GET /groups/{topic}/members 列表接口的每一项里压根没有 group_nickname 字段（只有
acs/display_name/mentionable/role/user_id），所以任何从成员列表里读昵称的写法，不管代码逻辑对不对，
都永远拿不到值——这个文件里原有的 MyNicknameEditor 也是同样的写法，同样存在这个问题（打开编辑器时
不会回填已保存的昵称）。

真正能读到昵称的是保存用的那个路径上对应的 GET：GET /groups/{topic}/members/me/profile，返回
{nickname, version, updated_at}（server/im_http.go:4776-4782）。im-api.js 里之前只封装了 PUT，
补上了对应的 getMyGroupProfile，两处读取（群资料页预览行、昵称编辑器回填）都改成用它。

实机验证：保存"测试昵称A"后重新打开群资料页，"群昵称"正确显示"测试昵称A"，"群名称"独立显示
"扫码测试群"。

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

src/lib/im-api.js

@@ -561,6 +561,11 @@ export const updateGroupMemberSettings = (topic, {pinned, mute_notifications}) =
   imPatch('/groups/' + encodeURIComponent(topic) + '/member-settings', {pinned, mute_notifications});
 export const updateMyGroupProfile = (topic, nickname, version) =>
   imPut('/groups/' + encodeURIComponent(topic) + '/members/me/profile', {nickname, version});
+// GET .../members/me/profile is the only endpoint that actually returns the saved nickname --
+// confirmed live that GET .../members (getGroupMembers below) never includes group_nickname on any
+// item, so anything reading the nickname from that list (as this file's own callers used to) will
+// always see it as unset even right after a successful save.
+export const getMyGroupProfile = topic => imGet('/groups/' + encodeURIComponent(topic) + '/members/me/profile');
 export const leaveGroup = topic => imPost('/groups/' + encodeURIComponent(topic) + '/leave', {});
 export const getGroupMembers = (topic, {query = '', cursor = '', role = '', eligibleRole = '', mentionable, muteEligible} = {}) =>
   imGet('/groups/' + encodeURIComponent(topic) + '/members' +