Never crash: Give the loupe a finite aspect for a zero-size crop

Count each side as at least one pixel.

docs/whats-new.md

@@ -2,6 +2,10 @@
 
 ## 2026-10-13
 
+### Reliability
+- A crop of no size (from an agent or a sidecar) no longer gives the Detail view a photo of no size to lay out
+  (its width over its height was 0 / 0, which stopped debug builds); it shows as a single pixel's colour.
+
 ### Slider drags on large raws
 - Dragging a slider no longer re-renders the edited photo's filmstrip thumbnail on every frame; it follows once you
   let go. On a 64 MP raw the first of those renders decoded the whole file again, which held up the loupe's