fix: a reply started in one channel thread no longer carries over to the next thread (#32490)

On a wide window the thread side panel keeps one Thread instance while
you open another thread, and initHandler reset the messages and typing
state for the new thread but not replyToMessage. The "Replying to"
banner stayed, and sending saved the new thread's message quoting a
message from the previous thread.

Clear replyToMessage in initHandler, as #31376 does for switching
channels.

src/lib/components/channel/Thread.svelte

@@ -49,6 +49,7 @@
 		searchAnchor = anchor;
 		messages = null;
 		top = false;
+		replyToMessage = null;
 
 		typingUsers = [];
 		typingUsersTimeout = {};