feat: render cancelled tool state in UI after user interrupt

feat: SSE keepalive, reconnection, and is_processing flag

fix: per-session state management for smooth task switching

fix: clear global activity state when switching away from a session

fix: protect system prompt in undo, scan backwards for dangling tool calls, gate input on SDK status

replace websocket transport with SSE

fix: restore activity status bar when switching back to session

fix: reconnect WS on session switch instead of deleting dead sessions

fix: approval UI not updating after approve click

feat: per-session WebSocket architecture for parallel chat sessions