fix: clear fetch error when zip is successfully loaded via drop

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

Author: hi-ogawa

src/index.html

@@ -238,6 +238,7 @@
           }
           setFileCount(count);
           setStatusMsg(`Cached ${count} files. Loading...`);
+          setFetchError(null);
           setLoaded(true);
         }