3 new modules:
- src/main/domain/pkce.ts: PKCE pair (S256) + state (RFC 7636)
- src/main/infra/loopback-server.ts: ephemeral 127.0.0.1:PORT redirect capture
- src/main/domain/twitch-oauth.ts: startLoginFlow / awaitAuthorizationCode /
exchangeCodeForToken / fetchTwitchUserInfo
Flow runs entirely scaffold-ready — IPC wiring + Client ID config and
shell.openExternal(authUrl) trigger come in a follow-on plan once the user
registers a Twitch dev app. 164 unit tests gruen.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
