feat(speakers): add people management and sample playback

[Optic00]

Summary

• add a dedicated People settings page for managing learned speaker profiles
• allow representative voice samples to be played without exposing source paths or requesting microphone access
• improve speaker-review ordering, multi-speaker handling, and dominant-channel decisions
• add T1 and T2 coverage for deletion, playback, privacy-safe failures, and persisted speaker decisions

Verification

• renderer unit tests, typecheck, lint (0 errors), and production build
• 210 focused Python tests
• PyInstaller backend build and MLX bundle guard (10/10 checks)
• speaker review T1: 28/28
• people management and speaker naming T2: 6/6

GitHub Actions passes all 11 checks on macOS and Windows, including the full Python, T1, T2, and bundle jobs.