* feat: add chat virtualization and turn grouping infrastructure
Implement VirtualMessageList with virtualization and overscan for smooth scrolling
Refactor MessageList to render only visible messages and pass scroll refs
Introduce TurnGroupingContext and provider to stabilize per-turn rendering and reduce re-renders
* feat: add web server session activity endpoint for visibility restore
Add /api/session-activity endpoint to expose tracked session activity
Use web server activity first when restoring visibility, with fallback to global status
Update server SSE to set session phase on activity events
* feat(chat): split TurnGroupingContext into UI/Streaming contexts
Add separate UI state and streaming contexts to reduce re-renders.
Skip animations for items visible in collapsed view when expanding.
Track shown parts in collapsed mode to fade in progressively
* feat(ui): cache turn groups with expand state and guard web activity
Add isExpanded to the turn cache key to trigger updates correctly
Expose isGroupExpanded from UI state so groups reflect expand/collapse
Limit web server session activity fetch to web runtime only
Bohdan Triapitsyn
f34027df10