Fix regenerate to persist image modifications to creative brief

- Update confirmedBrief.visual_guidelines when image regeneration succeeds
- Ensures subsequent Regenerate clicks use the updated brief with modifications
- Prevents reversion to original prompt parameters after modifications

Author: hunterjam

content-gen/src/app/frontend/src/App.tsx

@@ -341,6 +341,15 @@ function App() {
                     },
                   };
                   setGeneratedContent(responseData);
+                  
+                  // Update the confirmed brief to include the modification
+                  // This ensures subsequent "Regenerate" clicks use the updated visual guidelines
+                  const updatedBrief = {
+                    ...confirmedBrief,
+                    visual_guidelines: `${confirmedBrief.visual_guidelines}. User modification: ${content}`,
+                  };
+                  setConfirmedBrief(updatedBrief);
+                  
                   messageContent = parsedContent.message || 'Image regenerated with your requested changes.';
                 } else if (parsedContent.error) {
                   messageContent = parsedContent.error;