Add multi-select support to pmshare panel (#1779)

* Add multi-select support to share panel

- Replace single selection index with per-user selected flag
- Allow sending to multiple selected users
- Add sending state to prevent multiple clicks
- Update default selection logic to mark first user as selected

* 简化代码逻辑

* update

Signed-off-by: bggRGjQaUbCoE <githubaccount56556@proton.me>

---------

Signed-off-by: lesetong <oscarlbw@qq.com>
Co-authored-by: bggRGjQaUbCoE <githubaccount56556@proton.me>

lib/pages/follow/widgets/follow_item.dart

@@ -33,6 +33,7 @@ class FollowItem extends StatelessWidget {
                 mid: item.mid,
                 name: item.uname!,
                 avatar: item.face!,
+                selected: true,
               ),
             );
           } else {