- App provides HistoryHandle as context (both build modes; stacks stay
empty on the server) and passes it to persist::init_hydrated
- persist: init_hydrated starts undo tracking after the localStorage
load (first captured state = saved state, not mock); clear_all and
apply_import call undo::before_replace so they are undoable
- header: Отменить/Вернуть buttons with disabled states driven by the
reactive stack depths; Ctrl+Z / Ctrl+Shift+Z / Ctrl+Y via a window
keydown listener that skips editable targets (browser owns those)
- SCSS: disabled control buttons dim out
