feat: render cancelled tool state in UI after user interrupt

feat: SSE keepalive, reconnection, and is_processing flag

fix: properly close SDK message on error, show tool errorText

replace websocket transport with SSE

fix: buffer websocket events during disconnects and replay on reconnect

fix: merge consecutive assistant messages on page refresh

fix: show activity status for approval-flow tools and revert chip override

feat: resolve sandbox file paths and show tool description in status

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

feat: per-session WebSocket architecture for parallel chat sessions

feat: cooperative cancellation, session persistence, background generation

feat: merge HF Space improvements