Polish chat activity, sources, and stable disclosure geometry

[hamzamerzic]

Summary

• keep thinking and tool disclosures collapsed by default while persisting deliberate per-chat opens for the browser session
• remove redundant single-activity nesting and chevrons; give expanded command output a clear nested surface with existing copy controls
• render compact source pills only after the answer settles, without remote favicon requests
• preserve bounded lazy loading for thinking/tool previews and fetch full output only for explicit copy
• stabilize toggle, send, question-answer, Android viewport-growth, and tall-composer geometry

Production review

This incorporates the fixes found in the recent production chats and the production container review, plus the missing Android viewport-growth reservation that production had reproduced but not yet fixed.

Validation

• frontend: 1,732 library tests + 47 hook tests
• browser: 19 focused mobile/chat regressions
• backend: 2,657 pytest tests
• core-app, E2E harness, and static packager tests
• clean Node 24 production build + offline precache check
• full scripts/preship-gate.sh --full