Support configuring a custom HTTP header name for authentication
(e.g., X-API-Auth instead of Authorization) for Kanboard servers
with specific proxy configurations that use alternative auth headers.
- Add headerName field to apiTokenAuth and basicAuth structs
- Add WithAuthHeader() fluent method to Client
- Auth methods pass custom header name when creating auth structs
- Add tests for custom header with API token, basic auth, and custom user
Closes kanboard-9wa
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
