Step 1 of the gui/app.py + gui/preview.py structural refactor. Pure
move: the constant and predicate are now in their own module, and
preview.py grows a re-export shim at the bottom so existing imports
(app.py:1351 and the in-file class methods) keep working unchanged.
Shim is removed in commit 14 once importers update to the canonical
path. See docs/REFACTOR_PLAN.md for the full migration order.
