Adds the ability to delete chat sessions from the CoPilot interface:
Backend:
- Add DELETE /api/chat/sessions/{session_id} endpoint
- Returns 204 on success, 404 if not found or not owned
Frontend:
- Add delete button (trash icon) on hover for each chat session
- Add confirmation dialog before deletion
- Refresh session list after successful delete
- Clear current session if deleted
Closes: SECRT-1928
