perf: skip deep clone of message during streaming

During streaming, use a direct reference to the source message instead
of JSON.stringify comparison + JSON.parse deep clone on every token.
All mutation controls (edit, rate, feedback) are gated behind
message.done, so direct reference is safe during active streaming.
A deep clone is performed once when streaming completes.

Author: Algorithm5838

src/lib/components/chat/Messages/ResponseMessage.svelte

@@ -120,9 +120,18 @@
 	export let selectedModels = [];
 
 	let message: MessageType = JSON.parse(JSON.stringify(history.messages[messageId]));
+	let streaming = false;
+
 	$: if (history.messages) {
-		if (JSON.stringify(message) !== JSON.stringify(history.messages[messageId])) {
-			message = JSON.parse(JSON.stringify(history.messages[messageId]));
+		const source = history.messages[messageId];
+		if (source && !source.done) {
+			// During streaming: use direct reference to avoid costly serialization.
+			// All mutation controls (edit, rate, feedback) are gated behind message.done.
+			message = source;
+			streaming = true;
+		} else if (streaming || JSON.stringify(message) !== JSON.stringify(source)) {
+			message = JSON.parse(JSON.stringify(source));
+			streaming = false;
 		}
 	}