Add a single toggle switch to enable/disable sound effects in the Audio
settings. This provides a simpler alternative to full sound customization
while covering the most common use case.
- Add sound_effects.enabled config option (default: true)
- Add toggle switch in Audio settings tab
- Add helper methods to ConfigManager
- Update play_*_sound() to check enabled state
- Add tests for the new functionality
Closes#283
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>