Stage 3 Task 2. PKCE (S256) authorize URL, code exchange, and refresh with
rotated-refresh-token persistence + invalid_grant store-clear. axios/http and
token store injected for fully mocked, network-free unit tests.
Corrections C3.2 (require the operator's own client_id; no librespot public
client / :5588 default; buildAuthorizeUrl throws + isAuthorized false without
it) and C3.7 (delete the state->verifier map entry in a finally on every
terminal path) applied.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>