feat(slice-1): Vite/React shell + /api proxy; scripts/check.sh now fully green

Single tsconfig (no project-references split — avoids composite/noEmit friction for
the minimal shell); tsc typechecks src, vite loads its config via esbuild.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/tsconfig.json

@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "useDefineForClassFields": true,
+    "lib": ["ES2020", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "skipLibCheck": true,
+    "moduleResolution": "bundler",
+    "allowImportingTsExtensions": true,
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "noEmit": true,
+    "jsx": "react-jsx",
+    "strict": true
+  },
+  "include": ["src"]
+}