Fix scan/guide visibility with per-project preferences

- Store showScans/showGuides preferences per project so toggling
  persists when switching between projects
- Apply showGuides/showScans directly on guide and scan renderer groups
- Set projectId in both editor and viewer pages on mount

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/editor/app/viewer/[id]/page.tsx

@@ -28,6 +28,10 @@ export default function ViewerPage() {
   const [canShowGuides, setCanShowGuides] = useState(true)
   const setScene = useScene((state) => state.setScene)
 
+  useEffect(() => {
+    useViewer.getState().setProjectId(projectId)
+  }, [projectId])
+
   useEffect(() => {
     const loadContent = async () => {
       try {