android, desktop, ios: gate picker mention-id and max-reached banner by memberMentions

With the sticky cache, the picker's "currently bound member" highlight
and the showMaxReachedBox banner-suppression clause read mentions[name]
directly, so a stale cache entry made one row clickable (as a no-op) at
the MAX limit and could suppress the banner when the user is actually
adding a new mention. Both now gate the lookup by membership in the
capped memberMentions, restoring the pre-fix UX where at MAX all rows
are disabled and the banner shows when adding past the limit.

apps/ios/Shared/Views/Chat/Group/GroupMentions.swift

@@ -130,7 +130,7 @@ struct GroupMentionsView: View {
                 isVisible = true
                 mentionName = name
                 mentionRange = r
-                mentionMemberId = composeState.mentions[name]?.memberId
+                mentionMemberId = composeState.memberMentions[name] != nil ? composeState.mentions[name]?.memberId : nil
                 if !m.membersLoaded {
                     Task {
                         await m.loadGroupMembers(groupInfo)

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/chat/group/GroupMentions.kt

@@ -112,7 +112,7 @@ fun GroupMentions(
           isVisible.value = true
           mentionName.value = ft.format.memberName
           mentionRange.value = r
-          mentionMemberId.value = composeState.value.mentions[mentionName.value]?.memberId
+          mentionMemberId.value = if (mentionName.value in composeState.value.memberMentions) composeState.value.mentions[mentionName.value]?.memberId else null
           if (!chatModel.membersLoaded.value) {
             scope.launch {
               setGroupMembers(rhId, chatInfo.groupInfo, chatModel)
@@ -210,7 +210,7 @@ fun GroupMentions(
       },
     contentAlignment = Alignment.BottomStart
   ) {
-    val showMaxReachedBox = composeState.value.memberMentions.size >= MAX_NUMBER_OF_MENTIONS && isVisible.value && composeState.value.mentions[mentionName.value] == null
+    val showMaxReachedBox = composeState.value.memberMentions.size >= MAX_NUMBER_OF_MENTIONS && isVisible.value && mentionName.value !in composeState.value.memberMentions
     LazyColumnWithScrollBarNoAppBar(
       Modifier
         .heightIn(max = MAX_PICKER_HEIGHT)