[codex] Add FloodDiffusion online mesh streaming

[Xiangyue-Zhang]

Summary

• Adds a FastAPI proxy for the FloodDiffusion streaming API so Online mode can call an external GPU service instead of embedding model code in the frontend.
• Adds a client-side SMPL-H mesh avatar for streaming rotmats/transl with neutral SMPL-H static data.
• Adds Online controls for update text, pause/resume, stop, and restart while preserving Offline and Offline Audio flows.
• Keeps the viewer style aligned with main, adds camera follow, a stable larger floor, and fading motion trails.

Validation

• python3 -m py_compile app.py
• node --check static/js/main.js
• node --check static/js/avatars/smplh-avatar.js
• node --check static/js/avatars/avatar-factory.js
• Local browser smoke test at http://127.0.0.1:7872/?latest-main-online=6 with FloodDiffusion API on http://127.0.0.1:7870