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/capture/MeasurePanel.svelte

@@ -107,7 +107,7 @@
         <div><span>Scale</span><span>{state.results.scale.toFixed(2)} px/in</span></div>
       </div>
       {#if state.results.sourceBelowFloor}
-        <div class="warn">Source is below the 1600px floor for the inventory PNG. Retake a larger photo.</div>
+        <div class="warn">Source is below the 800px floor for the inventory PNG. Retake a larger photo.</div>
       {/if}
       {#if pngUrl && !state.results.sourceBelowFloor}
         <div class="png-wrap">

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;