Summary

Extends rawRequest in client.ts to support multipart/form-data
requests alongside the existing application/json behavior.

Changes

• Updated RequestOptions type in types.ts to accept FormData
  in addition to Record<string, unknown>
• Updated rawRequest in client.ts to check if data is a
  FormData instance and skip JSON stringification and manual
  Content-Type header in that case, allowing fetch to set the
  correct multipart/form-data header automatically

Motivation

Required groundwork for implementing the inventory upload endpoints
(related to #78). The Discogs inventory upload API expects
multipart/form-data with a CSV file, which the current
implementation doesn't support.

Testing

All existing tests pass — no current behavior is affected.