Add hook-based external volume controller

[balloob-travel]

Summary

• add a hook-based external volume backend that persists logical volume/mute state
• pass volume controllers into AudioStreamHandler via a protocol instead of selecting implementations there
• wire --hook-set-volume through CLI/settings and document the new behavior

Fixes #126

Testing

• uv run ruff check sendspin/audio_connector.py tests/test_audio_connector.py
• uv run pytest tests/test_audio_connector.py tests/test_hook_volume.py tests/test_hardware_volume.py