feat(#21): Gate B green/black sheet mask (TV-1, TV-2)

Pure client modules for TECH-SPEC sheet detect, chroma-key / black
threshold with border flood-fill, four knobs (live re-run), and mask
confirm hooks. Synthetic fixtures. U2Net never invoked.

TDD: red evidence in test-results/tv1-tv2-red.txt, then minimum impl.

Results:
TV-1 | pass | test-results/tv1-tv2-green.txt
TV-2 | pass | test-results/tv1-tv2-green.txt

frontend/vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/__tests__/**/*.test.ts'],
+  },
+});