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
Move 5 popout lifecycle methods (_open_fullscreen_preview,
_on_fullscreen_closed, _navigate_fullscreen, _update_fullscreen,
_update_fullscreen_state) and 4 state attributes (_fullscreen_window,
_popout_active, _info_was_visible, _right_splitter_sizes) into
gui/popout_controller.py.
Rename pass across ALL gui/ files: self._fullscreen_window ->
self._popout_ctrl.window (or self._app._popout_ctrl.window in other
controllers), self._popout_active -> self._popout_ctrl.is_active.
Zero remaining references outside popout_controller.py.
Extract build_video_sync_dict as a pure function for Phase 2 tests.
main_window.py: 2145 -> 1935 lines.
behavior change: none
