fix(chat): fix jump-to-oldest-unread scroll not reaching target message

- Pass initialUnreadCount to chat screens before markRead clears it
- Use two-phase scroll: jumpTo estimated offset to build lazy items,
  then ensureVisible for precise positioning
- Await ensureVisible before clearing scroll guard to prevent
  scrollToBottomIfAtBottom from overriding the animation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/channels_screen.dart

@@ -492,13 +492,18 @@ class _ChannelsScreenState extends State<ChannelsScreen>
             ],
           ),
           onTap: () async {
+            final unread =
+                connector.getUnreadCountForChannelIndex(channel.index);
             connector.markChannelRead(channel.index);
             await Future.delayed(const Duration(milliseconds: 50));
             if (context.mounted) {
               Navigator.push(
                 context,
                 MaterialPageRoute(
-                  builder: (context) => ChannelChatScreen(channel: channel),
+                  builder: (context) => ChannelChatScreen(
+                    channel: channel,
+                    initialUnreadCount: unread,
+                  ),
                 ),
               );
             }