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.

docs/CAPTURE.md

@@ -109,7 +109,7 @@ On Calculate, the app builds the inventory image from the approved mask:
 1. Keep slab pixels from the source photo. Removed pixels are transparent.
 2. Place the slab in a 3:4 portrait frame, centered.
 3. Scale so the widest and tallest points of the slab fill 80% of the frame. Extra margin where the slab is narrower.
-4. After that crop, shorter side ≥ 1600px when the source can supply it. Otherwise warn and require a new photo. No upscale from an undersized source.
+4. After that crop, shorter side ≥ 800px when the source can supply it. Otherwise warn and require a new photo. No upscale from an undersized source.
 5. Encode as PNG with alpha.
 
 ## Device draft
@@ -134,7 +134,7 @@ The first inventory photo is the measurement reference. Later inventory photos u
 | Alpha | Kept through encode and upload |
 | Aspect | 3:4 portrait |
 | Layout | Slab centered, 80% fill at the widest and tallest points |
-| Size | After crop, shorter side ≥ 1600px when the source can supply it |
+| Size | After crop, shorter side ≥ 800px when the source can supply it |
 
 Source below that floor: warn and require a new photo. No upscale from an undersized source.
 Editor preview of alpha is a light gray checkerboard. The storefront uses the theme background.

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;