fix: URL revoke race in exportSave

URL.revokeObjectURL() was called synchronously after a.click(), but
some browsers need the blob URL alive during download initiation.
Now delayed 1s via setTimeout to let the download start safely.
Fixes #63

game.js

@@ -2842,7 +2842,8 @@ function exportSave() {
     const ts = new Date().toISOString().slice(0, 10);
     a.download = `beacon-save-${ts}.json`;
     a.click();
-    URL.revokeObjectURL(url);
+    // Delay revoke to avoid race — some browsers need time to start the download
+    setTimeout(() => URL.revokeObjectURL(url), 1000);
     log('Save exported to file.');
 }