test(history): jsdom/renderHook lane + useBackToExit back-ladder tests

First DOM-capable vitest lane for the repo: default env stays node (pure lib helpers), a test opts
into jsdom per-file with a // @vitest-environment jsdom docblock; plugin-react wired for JSX/hook tests.
Adds jsdom + @testing-library/react devDeps and 3 back-ladder tests (LIFO peel, stacks over
useBackToClose, inactive layer is transparent) driven by direct popstate dispatch — deterministic,
no dependency on jsdom's async back(). Closes the deferred R2 component-test gap (useBackToExit).

frontend/package.json

@@ -37,6 +37,7 @@
   "devDependencies": {
     "@eslint/js": "9.39.1",
     "@playwright/test": "^1.61.1",
+    "@testing-library/react": "^16.3.2",
     "@types/node": "^26.1.1",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
@@ -47,6 +48,7 @@
     "eslint-plugin-react-hooks": "5.2.0",
     "eslint-plugin-react-refresh": "0.4.24",
     "globals": "16.5.0",
+    "jsdom": "^25.0.1",
     "knip": "6.27.0",
     "postcss": "^8.4.39",
     "prettier": "3.6.2",