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/contact/view.dart

@@ -47,7 +47,7 @@ class _ContactPageState extends State<ContactPage>
         actions: [
           IconButton(
             onPressed: () async {
-              UserModel? userModel = await Navigator.of(context).push(
+              final UserModel? userModel = await Navigator.of(context).push(
                 GetPageRoute(
                   page: () => FollowSearchPage(
                     mid: mid,