docs: add comprehensive encryption architecture documentation (#50)

- Create ENCRYPTION.md with detailed architecture documentation
  - Architecture overview with mermaid diagrams
  - Security model and trust boundaries
  - Implementation details with code examples
  - Usage guide for developers
  - Threat model analysis
  - Cryptographic specifications
  - Testing guidelines

- Create SECURITY.md with security best practices
  - Developer guidelines (key management, error handling, etc.)
  - User guidelines (URL sharing, PIN selection, etc.)
  - Incident response procedures
  - Security checklist for releases

- Create R2_DIRECT_ACCESS.md for future optimization
  - Direct browser-to-R2 upload/download architecture
  - Presigned URL implementation guide
  - Security considerations for direct access
  - Migration strategy from Worker proxy

- Enhance inline code documentation
  - Add detailed module documentation to crypto.ts
  - Add security properties documentation to auth.ts
  - Add binary format structure to binary.ts
  - Add usage flow documentation to crypto-utils.ts

- Update README.md to reference new documentation

Closes #41

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

Co-authored-by: Claude <noreply@anthropic.com>

Author: nullcoder

README.md

@@ -119,6 +119,8 @@ npm run deploy
 ## 📖 Documentation
 
 - [Technical Specification](docs/SPEC.md) - Detailed architecture and implementation details
+- [Encryption Architecture](docs/ENCRYPTION.md) - In-depth encryption documentation
+- [Security Best Practices](docs/SECURITY.md) - Security guidelines for developers and users
 - [AI Development Guide](CLAUDE.md) - Guidelines for AI-assisted development
 - [Implementation TODO](docs/TODO.md) - Development roadmap and progress tracking
 - [Contributing Guide](CONTRIBUTING.md) - How to contribute to the project