* feat(web): download attachments in frontend
* fix: proxy pico media and force svg downloads
* feat(web): hide ephemeral media refs from persisted session history
- move chat controller, state, protocol, history, and websocket logic into a dedicated chat feature module
- improve chat reconnection, session hydration, and send gating based on actual websocket state
- preserve gateway status during transient SSE disconnects and update stop state immediately
- generate wss websocket URLs behind HTTPS proxies and add backend tests for forwarded proto handling
