feat: enhance demo app with file upload and improved notifications

Improve user experience with better file management and visibility:
- Add segregated upload modes (text/file) with toggle UI
- Display full copyable addresses for uploaded items
- Implement file upload from device storage
- Add manual download section with text/file mode selection
- Replace status card with bottom-anchored Snackbar notifications
- Save downloaded files to Downloads folder using MediaStore API

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: b-zee