FEATURE: Show unread in sidebar for unread channel threads (#22342)

This commit makes it so that when the user has unread threads
for a channel we show a blue dot in the sidebar (or channel index
for mobile/drawer).

This blue dot is slightly different from the channel unread messages:

1. It will only show if the new thread messages were created since
   the user last viewed the channel
2. It will be cleared when the user views the channel, but the threads
   are still considered unread because we want the user to click into
   the thread list to view them

This necessitates a change to the current user serializer to also
include the unread thread overview, which is all unread threads
across all channels and their last reply date + time.

plugins/chat/plugin.rb

@@ -237,6 +237,17 @@ after_initialize do
 
   add_to_serializer(:current_user, :chat_channels) do
     structured = Chat::ChannelFetcher.structured(self.scope)
+
+    if SiteSetting.enable_experimental_chat_threaded_discussions
+      structured[:unread_thread_overview] = ::Chat::TrackingStateReportQuery.call(
+        guardian: self.scope,
+        channel_ids: structured[:public_channels].map(&:id),
+        include_threads: true,
+        include_read: false,
+        include_last_reply_details: true,
+      ).thread_unread_overview_by_channel
+    end
+
     Chat::ChannelIndexSerializer.new(structured, scope: self.scope, root: false).as_json
   end