[rfprod]

• add TerminalBackend class with ncurses init, dark and light themes, and render loop;
• handle SIGWINCH resize and SIGINT interrupts via atomic flags;
• route print, spinner, progress, input buffer, and redraw calls through the backend;
• move Layout struct and message type constants (normal, error, success) into the backend;
• hold a unique_ptr to TerminalBackend in UserInterface, remove ncurses state;
• replace ui_running() with backend_running() across all components;
• use a 100 ms getch timeout in next_prompt so it can detect backend shutdown;
• replace std::exit with end_session and ExitRequest;
• add ExitRequest with an exit code, reason, and noexcept code() and what() accessors;
• add utils/exit_request.hpp so argument handlers can throw ExitRequest;
• wrap session logic in main in a try block that catches ExitRequest and returns its code;
• route fatal Qdrant, Ollama, CURL, and system errors through end_session(EXIT_FAILURE);
• remove std::exit from check_ollama, process_response, and query_model_list;
• keep HttpException for non-fatal HTTP errors when fail_on_http_error is false;
• add #pragma once to http_utils to prevent duplicate definitions;
• replace EXPECT_EXIT death tests with direct calls that check the end_session code;
• update the UserInterface mock to implement backend_running() instead of ui_running();