[codex] isolate model selection per thread

[musnows]

Summary

• Store composer model/provider selections per thread instead of only globally.
• Restore the active thread's saved model selection when models load or when switching threads.
• Keep global Kun default updates limited to the no-active-thread picker state.

Root cause

The composer model state was backed by a single global store/localStorage entry, so selecting a model in one conversation changed the picker state used by other conversations.

Validation

• npm test -- src/renderer/src/store/chat-store-helpers.test.ts src/renderer/src/store/chat-store-app-actions.test.ts
• npm run typecheck