feat: implement Web Crypto API encryption/decryption utilities

[nullcoder]

Description

Implement the core encryption module using Web Crypto API for AES-GCM encryption/decryption that works in Cloudflare Workers edge runtime.

Tasks

• Create lib/crypto.ts with the following functions:
  • generateEncryptionKey() - Generate 256-bit AES key
  • exportKey(key: CryptoKey) - Export key to base64url
  • importKey(keyString: string) - Import base64url key
  • encrypt(data: Uint8Array, key: CryptoKey) - AES-GCM encryption with fresh IV
  • decrypt(encryptedData: Uint8Array, key: CryptoKey) - AES-GCM decryption
• Implement proper error handling with custom error classes
• Add JSDoc documentation for all functions
• Create comprehensive unit tests

Technical Requirements

• Use Web Crypto API (no Node.js crypto)
• 256-bit AES-GCM encryption
• 12-byte IV (crypto.getRandomValues)
• Base64url encoding for URL safety
• Edge runtime compatible

Acceptance Criteria

• All crypto operations use Web Crypto API
• Tests pass in edge runtime environment
• Proper error handling for invalid keys/data
• Type-safe implementation with TypeScript