fix(feedback): show the operator reply only on the player's latest message

A reply was bound to 'the latest message that has a reply', so after the player
read a reply and sent a new (unanswered) message, the old reply kept showing as
'Ответ на ваше последнее сообщение'. Bind it to the single most-recent message
instead: sending any new message immediately drops the previous reply (the new
message has no reply yet), well before the one-week window. Client clears the
reply optimistically on submit; the mock mirrors it; inttest covers the case.

ui/src/lib/mock/client.ts

@@ -418,6 +418,7 @@ export class MockGateway implements GatewayClient {
   }
   async feedbackSubmit(_body: string, _attachment: Uint8Array | null, _attachmentName: string, _channel: string): Promise<void> {
     this.feedbackPending = true;
+    this.feedbackReply = null; // a new message: the previous operator reply no longer shows
   }
   async feedbackGet(): Promise<FeedbackState> {
     this.feedbackReplyUnread = false; // fetching the screen delivers (reads) the reply