Step 13 of the gui/app.py + gui/preview.py structural refactor —
final move out of app.py. The four entry-point helpers move together
because they're a tightly-coupled cluster: run() calls all three of
the others (_apply_windows_dark_mode, _load_user_qss,
_BASE_POPOUT_OVERLAY_QSS). Splitting them across commits would just
add bookkeeping overhead with no bisect benefit.
app_runtime.py imports BooruApp from main_window for run()'s
instantiation site, plus Qt at module level (the nested
_DarkArrowStyle class inside run() needs Qt.PenStyle.NoPen at call
time). Otherwise the four helpers are byte-identical to their
app.py originals.
After this commit app.py is just the original imports header + log
+ the shim block — every entity that used to live in it now lives
in its canonical module. main_gui.py still imports from
booru_viewer.gui.app via the shim (`from .app_runtime import run`
re-exports it). Commit 14 swaps main_gui.py to the canonical path
and deletes app.py.
