feat: correct uneven lighting and pick polarity automatically

Fit the background brightness from the photo border and subtract it, so
side-lit photos threshold cleanly. The clicked pixel (or, without a click,
the border) decides whether the piece is darker or lighter, replacing the
invert checkbox. Adds the MDF-on-wood-floor photo as a regression fixture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/app-state.svelte.ts

@@ -22,7 +22,6 @@ export class AppState {
   scalePoints = $state<Point[]>([]);
   lengthMm = $state<number | null>(null);
   threshold = $state(DEFAULT_THRESHOLD);
-  invert = $state(false);
   seed = $state<Point | null>(null);
   marginMm = $state(-2);
   mode = $state<'auto' | 'manual'>('auto');
@@ -45,7 +44,7 @@ export class AppState {
       return shapeFromPolygons(cv, image.width, image.height, { outer: this.manualOuter, holes: this.manualHoles }, this.marginPx);
     }
     return detect(cv, image, {
-      threshold: this.threshold, invert: this.invert, seed: this.seed, marginPx: this.marginPx,
+      threshold: this.threshold, seed: this.seed, marginPx: this.marginPx,
       minHolePx: this.pxPerMm ? MIN_HOLE_MM * this.pxPerMm : 0,
     });
   });