UAT-7: fix auto key selection — prefer key that removes more background

Root cause: auto selected the key with MORE kept pixels, which always
picked the green key (it keeps 100% of dark non-green backgrounds).
The black key correctly isolates the slab on dark surfaces.

Fix:
- auto now prefers the key with FEWER kept pixels (removed more bg)
  when both are non-empty
- green key has a greenness pre-filter: non-greenish pixels pass
  through (kept) so the black key can handle them, instead of the
  green key keeping 100% of a dark surface
- added 3 regression tests: dark-bg auto selection, green-sheet auto
  selection, green-key passthrough on non-green surfaces

106/106 unit tests green.

frontend/tests/unit/mask.test.js

@@ -96,6 +96,44 @@ describe('keySheet', () => {
     // 12x8 = 96 pixels
     expect(maskStats(mask).kept).toBe(96);
   });
+
+  it('green key passes non-green pixels through (black key handles them)', () => {
+    // Real slab photos (sample_01..06) have a dark blue-gray background
+    // (e.g. rgb 26,27,29) with a bright warm slab (e.g. rgb 250,200,140).
+    // The green key is designed to key out a GREEN sheet only. On a dark
+    // (non-green) surface it passes pixels through (keeps them) so that
+    // the black key can remove them. The green key alone does NOT remove
+    // a dark background — that is the black key's job. Auto selects the
+    // black key in this case (see the auto test below).
+    const darkBg = solid(12, 8, 26, 27, 29);
+    withRect(darkBg, 12, 2, 2, 8, 4, 250, 200, 140);
+    const mask = keySheet(darkBg, { sheet: 'green', sensitivity: 50 });
+    const stats = maskStats(mask);
+    // Green key on dark bg: all pixels are non-greenish, so all are kept.
+    // This is BY DESIGN — the green key is a no-op on non-green surfaces,
+    // and auto will select the black key instead.
+    expect(stats.kept).toBe(96);
+  });
+
+  it('auto selects black key over green key on a dark-background photo', () => {
+    // Real photo: dark bg + bright slab. Green key keeps ~everything (bad).
+    // Black key keeps only the slab (good). Auto must pick black.
+    const darkBg = solid(12, 8, 26, 27, 29);
+    withRect(darkBg, 12, 2, 2, 8, 4, 250, 200, 140);
+    const mask = keySheet(darkBg, { sheet: 'auto', sensitivity: 50 });
+    const stats = maskStats(mask);
+    // Should keep only the slab (32 px), not the whole frame (96 px).
+    expect(stats.kept).toBe(32);
+  });
+
+  it('auto selects green key over black key on a green-sheet photo', () => {
+    // Green sheet + brown slab. Green key keeps slab only. Black key may
+    // keep slab + some sheet. Auto must pick green (fewer kept = better).
+    const photo = GREEN_PHOTO();
+    const mask = keySheet(photo, { sheet: 'auto', sensitivity: 50 });
+    const stats = maskStats(mask);
+    expect(stats.kept).toBe(32);
+  });
 });
 
 // --- maskStats -------------------------------------------------------------------