- Fix stale configRef in useConfig so saves include the latest state.
- Preserve Pi models.json fields (headers, compat, cost, maxTokens,
thinkingLevelMap, authHeader, modelOverrides, provider name) across edits.
- Add Pi settings.json I/O and update defaultProvider/defaultModel when
activating a provider so Pi actually picks up the switch.
- Add round-trip and settings serialization tests.