Move 10 media loading methods (_on_post_activated, _on_image_done,
_on_video_stream, _on_download_progress, _set_preview_media,
_prefetch_adjacent, _on_prefetch_progress, _auto_evict_cache,
_image_dimensions) and _prefetch_pause state into
gui/media_controller.py.
Extract compute_prefetch_order as a pure function for Phase 2 tests.
Update search_controller.py cross-references to use media_ctrl.
main_window.py: 2525 -> 2114 lines.
behavior change: none