## Why
TUI preferences and persistence paths belong to the local client, while thread
configuration and account requirements may come from the active app server.
Keeping both in the same resolved `Config` can replace live local preferences
when a thread is resumed, forked, reconnected, or switched.
## What changed
- Add `LocalSettings` as the TUI-owned source for UI, history, notice, and local
persistence settings, and preserve it across session lifecycle transitions.
- Write preference changes to the selected user config file and reload local
settings from disk when changing roots.
- Use the app server's account response for authentication-dependent onboarding
and status UI instead of the local model-provider configuration.
- Retain platform family and OS metadata reported by remote app servers.
## Testing
- Cover local setting defaults, overrides, persistence destinations, reloads,
and preservation across widget replacement and root switching.
- Cover remote platform metadata and server-controlled authentication UI.
GitOrigin-RevId: dbc2965fb1d6cbecb3f973f6133f3dcd4467b753
9d57be71ba