Fix recording and playback issues

- Fix download filename handling in converter and export functions
- Make updateRecordCanvas return Promise to handle async loadFromJSON correctly
- Add setCoords() call in setObjectValue to fix object selection after updates
- Update recorder initialization and animation frame handling

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/js/converter.js

@@ -94,7 +94,7 @@ function PostBlob(blob) {
   const a = document.createElement('a');
   a.style.display = 'none';
   a.href = url;
-  a.download = name;
+  a.download = blob.name || 'video';
   document.body.appendChild(a);
   a.click();
   recording = false;