feat: add vibe-aware continuous playback

[kylejschultz]

Goal

Keep listening going when the current track ends by selecting a suitable next track from the local library, instead of stopping playback.

Desired behavior

• When a track finishes and no explicit queue item follows it, Prism selects and plays another track that fits the current listening context.
• Matching should consider available signals such as genre, artist, album, and related metadata.
• Avoid immediate repeats and favor variety while maintaining a coherent vibe.

Acceptance criteria

• Continuous playback starts a suitable next track when normal playback would otherwise end.
• Explicit user queue and playlist ordering continue to take priority.
• The recommendation pool excludes the just-played track and handles a small or sparse library gracefully.
• The selection rationale is modular and tunable, so matching weights/signals can evolve without rewriting playback flow.
• Users can turn the behavior off.

Discovery questions

• Which local metadata signals produce the best first-pass matches?
• How should album context differ from artist, genre, or playlist context?
• What history window best prevents repetition without exhausting smaller libraries?

Out of scope

• A network-hosted recommendation service.
• Changing the behavior of an explicitly ordered queue or playlist.