perf: shallow spread in streaming fast path

Author: Algorithm5838

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

@@ -65,7 +65,7 @@
 		const source = history.messages[messageId];
 		if (source) {
 			if (message.content !== source.content || message.done !== source.done) {
-				message = structuredClone(source);
+				message = { ...source };
 			} else if (JSON.stringify(message) !== JSON.stringify(source)) {
 				message = structuredClone(source);
 			}

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

@@ -126,7 +126,7 @@
 			// Fast path: O(1) check on the fields that change most often (content during streaming, done at end)
 			// Avoids 2x O(n) JSON.stringify calls that are always true during streaming anyway
 			if (message.content !== source.content || message.done !== source.done) {
-				message = structuredClone(source);
+				message = { ...source };
 			} else if (JSON.stringify(message) !== JSON.stringify(source)) {
 				// Slow path: full comparison for infrequent changes (sources, annotations, status, etc.)
 				message = structuredClone(source);