content: Update Mention widget to render user group mentions dynamically

Fixes #1259.

Look up current user group name from PerAccountStore when rendering
mentions, while respecting "isSilent" flag. This ensures mentions
display correctly even if the user group name has been changed since
the message was sent. Falls back to original HTML text when user
group not found. Added 'user group mention dynamic name resolution'
tests group.

Author: Ruhdee

lib/widgets/content.dart

@@ -1206,6 +1206,12 @@ class Mention extends StatelessWidget {
       if (user case User(:final fullName)) {
         nodes = [TextNode(node.isSilent ? fullName : '@$fullName')];
       }
+    } else if (node case UserGroupMentionNode(:final userGroupId)) {
+      final userGroup = store.getGroup(userGroupId);
+      if (userGroup case UserGroup(:final name)) {
+        // TODO(#1260) Get display name for system groups using localization
+        nodes = [TextNode(node.isSilent ? name : '@$name')];
+      }
     }
     return Container(
       decoration: BoxDecoration(

test/widgets/content_test.dart

@@ -1111,6 +1111,45 @@ void main() {
         check(find.text('@New Name')).findsNothing();
       });
     });
+
+    group('user group mention dynamic name resolution', () {
+      Future<void> prepare({
+        required WidgetTester tester,
+        required String html,
+        List<UserGroup>? userGroups,
+      }) async {
+        final initialSnapshot = eg.initialSnapshot(realmUserGroups: userGroups);
+        await prepareContent(tester,
+          wrapWithPerAccountStoreWidget: true,
+          initialSnapshot: initialSnapshot,
+          plainContent(html));
+      }
+
+      testWidgets('resolves current user group name from store', (tester) async {
+        await prepare(
+          tester: tester,
+          html: '<p><span class="user-group-mention" data-user-group-id="186">@old-name</span></p>',
+          userGroups: [eg.userGroup(id: 186, name: 'new-name')]);
+        check(find.text('@new-name')).findsOne();
+        check(find.text('@old-name')).findsNothing();
+      });
+
+      testWidgets('falls back to original text when user group not found', (tester) async {
+        await prepare(
+          tester: tester,
+          html: '<p><span class="user-group-mention" data-user-group-id="999">@Unknown Group</span></p>');
+        check(find.text('@Unknown Group')).findsOne();
+      });
+
+      testWidgets('handles silent mentions correctly', (tester) async {
+        await prepare(
+          tester: tester,
+          html: '<p><span class="user-group-mention silent" data-user-group-id="186">old-name</span></p>',
+          userGroups: [eg.userGroup(id: 186, name: 'new-name')]);
+        check(find.text('new-name')).findsOne();
+        check(find.text('@new-name')).findsNothing();
+      });
+    });
   });
 
   Future<void> tapText(WidgetTester tester, Finder textFinder) async {