fix(chat): normalize session creation errors

[wunitb]

Summary

• recursively normalize structured session-creation errors from message, detail, and error envelopes
• prevent literal [object Object] from reaching Group Chat toasts
• route Group session creation through the shared error normalizer with an unknown catch boundary

Root cause

The backend already returns structured FastAPI errors such as {"detail":{"code":"...","message":"..."}}. Some runtime/client paths can rethrow an object or place a nested object in Error.message; GroupsPage read error.message directly, so the UI rendered JavaScript object coercion instead of the backend diagnostic. Backend create_group_session and its transaction path are covered and no repository defect blocking valid session creation was reproduced.

Verification

• frontend contract suite: 124 passed
• TypeScript + Vite production build: passed
• focused error tests cover nested objects, malformed Error.message, and [object Object] fallback

Deployment

The branch is rebased onto current upstream main at e190c982. The same change is merged into the deployable fork wunitb/Clawith as b9ba45ec. Upstream merge/deploy is still required before installations tracking dataelement/Clawith receive the fix.

[wunitb]

Rebased the tested fix onto current main (e190c982). The Group Chat create-session backend path is valid in repository tests; the confirmed defect is UI object coercion hiding the structured backend message. Full frontend contract suite: 124 passed; production build passed. Please merge/deploy to replace COULD NOT CREATE THE SESSION — [OBJECT OBJECT] with the real diagnostic.