0.0.20

refactor: AI settings → pi-web agent config

- Remove AiProtocol, API key, reasoning effort, safeStorage
- AiConfig simplified to { piWebUrl, defaultModel }
- AiService fetchModels from {piWebUrl}/api/models (modelList)
- AiService testConnection: simple GET health check
- AiSettings UI: URL + test connection + model browser
- SongInfoService reads URL/model from AiConfig via getter