Introduce a /stop API handler (api/stop.py) and its DOX to cancel an AgentContext run without deleting the context; it clears pause state, progress and logs an info step. Add tests for stopping behavior, chat working animation, and sidebar timestamp/spacing (tests/*). Update helpers/git._format_git_timestamp to emit UTC timestamps without a timezone suffix and add a test for it. Implement stop-related UI/UX: input-store.js gains a stop state, activateSendButton(), and stopAgent() to call /stop; chat-bar-input.html updates the send button markup, aria-label and stop styles. Update sidebar/chat list CSS to add the working-bubble animation, adjust chat row layout and action-button visibility, and left-sidebar/bottom styling (nowrap timestamp). Modify messages.js to hide kvps.finished from display and complete the active process group when finished. Minor welcome-composer and documentation updates to reflect these behaviors.
