fix: prevent local dropped models from being overwritten on refresh

feat: improve mobile touch controls and add local file drop support

style: translate comment to english

perf: use dynamic chunk size for large files

perf: increase upload concurrency to 6

feat: implement concurrent chunked upload and JYSM prefix URL routing

refactor: extract UploadOverlay into separate component

fix: vertical centering of upload UI when world is empty

feat: implement real upload progress bar with XMLHttpRequest

feat: add uploading progress overlay during drag-and-drop

Initial commit: SparkSplat with proper UTF-8 LFS