CI installs httpx + Pillow + pytest but not PySide6. The Phase C
tests import pure functions from controller modules, which had
top-level PySide6 imports (QTimer, QPixmap, QApplication, QMessageBox).
Move these to lazy imports inside the methods that need them so the
module-level pure functions remain importable without Qt.
Move 26 bookmark/save/library/batch/blacklist methods and _batch_dest
state into gui/post_actions.py. Rewire 8 signal connections and update
popout_controller signal targets.
Extract is_batch_message and is_in_library as pure functions for
Phase 2 tests. main_window.py: 1935 -> 1400 lines.
behavior change: none
