feat(rn): [P1] Image attachments in composer

[liuyidi]

Summary

Pick images from gallery and send with chat turns; show thumbnails in thread.

WebUI reference

• webui/src/components/thread/ThreadComposer.tsx attachment / image payload path

RN landing

• FloatingChatComposer.tsx + expo-image-picker
• (tabs)/index.tsx send path includes attachments
• Bubble thumbnail display

Acceptance

• Pick image → send → assistant can respond from image
• Over-limit / type errors show user-facing hint

Depends

None (independent of settings pages)