QGraphicsView for the viewport, with cuts and the content rectangle
manipulated by hit-testing in the view rather than as movable items —
the geometry is normalised, so what the screen shows and what the
renderer uses are the same numbers at a different zoom.
Cuts are edited as polylines: double-click adds one, drag moves it,
Ctrl-click inserts a vertex, right-click deletes a vertex or the whole
cut. That is how a straight cut becomes the stepped cut Engel needs.
Slice boundaries are drawn, not just cut lines, so a trim anomaly is
visible before export rather than after. Discarded slices are shaded.
Pages preview at 1800px regardless of source resolution, cached per
page, because re-reading a 4959x7017 vector render on every slider move
is unusable.
Autosave is debounced at 800ms and also fires on close.
Closes#12Closes#14Closes#15Closes#16Closes#17Closes#18Closes#19Closes#20Closes#26
