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
Move _toggle_privacy and its lazy state (_privacy_on, _privacy_overlay,
_popout_was_visible) into gui/privacy.py. Rewire menu action, popout
signal, resizeEvent, and keyPressEvent to use the controller.
No behavior change. main_window.py: 3111 -> 3068 lines.
