web: compare the same frame rendered twice, whatever its geometry

The split used to paint the photo file beside the render, so it only lined up
while nothing had moved: a turn, a straighten, a printed frame and the halves
were two different pictures. The app now renders the same frame twice — once
through the look, once through the neutral stock — and the left of the bar is
that second copy. Rotation, straighten, crop and frame land on both halves by
construction, so the CSS that tried to map the crop onto the file goes away.

The toggle lives in the app now, which is what knows how to ask for the extra
render; it is only asked for while the split is up. The layer waits for that
copy rather than flashing the raw file, whose geometry is already wrong.

docker/frontend/src/styles/app.css

@@ -284,9 +284,6 @@ input[type="range"] { width: 100%; accent-color: var(--accent); }
   cursor: ew-resize;
   touch-action: none;
 }
-/* The window the original is looked at through: the render's own shape, so a
-   crop can slide and scale the photo behind it and still line up. */
-.compare-window { position: absolute; inset: 0; overflow: hidden; }
 .compare-orig { position: absolute; inset: 0; width: 100%; height: 100%; box-shadow: none; }
 .compare-bar {
   position: absolute;