Add download progress tracking to createBlobUrlViewer to fix incorrect
progress display for large files. Changes:
- Add xhr.onprogress handler to track download bytes
- Use 64-bit float division to avoid 32-bit integer overflow
- Show progress bar UI for files >10MB with percentage
- Add CSS styles with dark theme support
The progress calculation now correctly handles files larger than 2GB
by using JavaScript's native double-precision floats instead of
operations that might truncate to 32-bit integers.
Fixes#82
BillionClaw
5eb7b31740