Adds api.put helper, ListenBrainz API module with query/mutation
helpers, a /settings route with token + enable/disable UI, and 5
tests. Adds Settings to the Shell nav.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Wraps fetch, parses JSON, and throws ApiError{code,message,status}
on non-2xx. Degrades to {code:'unknown'} when the error body is not
the expected envelope shape.
bvandeusen