OpenQuack v2.0.0-alpha.16

Upgrade flow (SPEC-011 phase B.5): Homebrew users who click Upgrade no longer see a Terminal window. A detached script runs brew upgrade --cask openquack in the background, relaunches the duck when done, and the running instance exits cleanly after ~1.5 s. The banner shows a spinner while it's installing.

Custom dictionary default reverted: The Whisper-bias seed list shipped in alpha.15 is pulled back. WhisperKitEngine.transcribe() was re-encoding prompt tokens on every short clip without caching — SPEC-032 filed for the offline-path parity fix. Custom dictionary still works; just no pre-filled default until the cache lands.