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/tsconfig.json

@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "noEmit": true,
+    "skipLibCheck": true,
+    "types": ["vitest/globals"],
+    "lib": ["ES2022"]
+  },
+  "include": ["src/**/*.ts"]
+}