fix: use currently selected model when redoing messages

When redoing a 'proceed with build' message, the system now uses the
currently selected model from the dropdown instead of the original
message's model. This allows users to switch models before redoing.

chat/public/builder.js

@@ -360,7 +360,9 @@ async function redoMessage(msg, session) {
     }
 
     if (planContent) {
-      await redoProceedWithBuild(planContent, msg.model);
+      // Use null to force redoProceedWithBuild to use the currently selected model
+      // instead of the original message's model
+      await redoProceedWithBuild(planContent, null);
     } else {
       setStatus('Cannot redo: plan content not available');
     }