v1.18.1 — Twitch sign-in is live

A real Twitch application is now registered and its Client ID is baked in — "Connect Twitch Account..." (added in v1.18.0) is no longer disabled. Verified against Twitch's real API before shipping: a live device-code request was made and Twitch returned a genuine code and verification link. See CHANGELOG.md for details. Pre-release: the connection to Twitch itself is proven, but the human sign-in step and a real live alert trigger still need Harvey's testing pass.