Put a user message's copy button beside its timestamp

The copy button sat on its own row above the group's time, costing a
line of vertical space for nothing. It now shares that row, left of the
timestamp, the way a bot message's actions already do. A file-only
message leaves no gap: only messages with text get a button, picked by
copyableMessages.

src/pages/admin/bots/[bot_id]/(partials)/user-message-copy.tsx

@@ -0,0 +1,15 @@
+import CopyMessageButton from "./copy-message-button";
+
+type Props = {
+    text: string;
+};
+
+/**
+ * One message's copy button, in the row under a user group beside its
+ * timestamp. A message with no text renders nothing at all.
+ */
+export default function UserMessageCopy({ text }: Props) {
+    if (!text.trim()) return null;
+
+    return <CopyMessageButton text={text} />;
+}