feat(rn): [P0] Composer model picker

[liuyidi]

Summary

In-chat model switcher so users need not open Settings.

WebUI reference

• webui/src/components/thread/ThreadComposer.tsx (composer-model-picker)

API

Same activate endpoints as Models settings; current model from bootstrap / MinibotClientContext.modelName.

RN landing

• src/components/chat/FloatingChatComposer.tsx + ActionSheet/BottomSheet
• Optional: ModelPickerSheet.tsx
• Read useMinibot() modelName + client

Acceptance

• Works on empty and active sessions
• Next turn uses the newly activated model

Depends

• feat(rn): [P0] Models settings page (reuse settings fetch / activate)