fix: add Express back for reliability and improve wallet.ts type safety

- Add express and @types/express to dependencies for production reliability
  * Battle-tested error handling and middleware ecosystem
  * All MCP SDK examples use Express for HTTP transport
  * Minimal overhead (~50KB) with significant reliability gains
- Fix wallet.ts to use viem's documented HDAccount/PrivateKeyAccount types
  * Use getHdKey() method for extracting private key from mnemonic accounts
  * Proper type checking instead of unsafe 'as any' casting
  * Better error messages for debugging
- Update bun.lock with express dependencies

This ensures all MCP transports (stdio, StreamableHTTP) are production-ready.

Author: claude