fix(capture): lower inventory PNG floor to 800px

Product call for the WooCommerce redo. Calculate accepts
sources whose post-crop shorter side is at least 800px.
Warn copy and CAPTURE.md match. No upscale still holds.

frontend/src/png/inventory.ts

@@ -1,6 +1,6 @@
 import type { Mask, Source } from '../capture/capture.types';
 
-const FLOOR = 1600; // shorter side, px
+const FLOOR = 800; // shorter side, px
 const FILL = 0.8; // 80% fill at the widest and tallest points of the slab
 const ASPECT_W = 3;
 const ASPECT_H = 4;