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/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "slabuploader-frontend",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "devDependencies": {
+    "typescript": "^5.6.3",
+    "vitest": "^2.1.4"
+  }
+}