Password-Protected File Sharing Release

- Complete cfpu enhancement with -p password flag
- Cloudflare Workers + R2 + KV integration
- Secure password hashing and verification
- Dual-mode operation: public by default, password-protected on demand
- Production-ready implementation with proper error handling

Features:
- cfpu file.txt (public upload)
- cfpu file.txt -p password (protected upload)
- Beautiful password protection UI
- Secure password storage using SHA-256 hashing
- Remote storage configuration for production use