tooling: add ESLint 9 flat config and a typecheck script

- eslint.config.mjs: flat config built on eslint-config-next's flat array
  (core-web-vitals + typescript + Next plugin), with build/generated artifacts
  ignored and the aggressive react-hooks v6 React-Compiler rules downgraded to
  warnings so `pnpm run lint` passes clean (0 errors, 88 warnings of burn-down
  debt) instead of failing. Previously `eslint .` failed outright — no config
  and no eslint binary were installed.
- Install eslint, @eslint/js, eslint-config-next, typescript-eslint, globals as
  devDeps (dev-only; prod Docker build is unaffected).
- package.json: add `typecheck` (tsc --noEmit) and `test:run` scripts.
- app/not-found.tsx: escape apostrophes (react/no-unescaped-entities).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

package.json

@@ -7,7 +7,9 @@
     "build": "next build",
     "start": "next start",
     "lint": "eslint .",
+    "typecheck": "tsc --noEmit",
     "test": "vitest",
+    "test:run": "vitest run",
     "test:ui": "vitest --ui",
     "test:integration": "vitest run --config vitest.integration.config.ts",
     "test:e2e": "playwright test",
@@ -71,6 +73,7 @@
     "zod": "^3.24.1"
   },
   "devDependencies": {
+    "@eslint/js": "^9.39.5",
     "@playwright/test": "^1.40.0",
     "@tailwindcss/postcss": "^4.2.0",
     "@testing-library/jest-dom": "^6.1.5",
@@ -80,12 +83,16 @@
     "@types/react-dom": "19.2.3",
     "@vitejs/plugin-react": "^4.2.1",
     "@vitest/ui": "^1.0.4",
+    "eslint": "^9.39.5",
+    "eslint-config-next": "16.1.6",
+    "globals": "^15.15.0",
     "jsdom": "^23.0.1",
     "postcss": "^8.5",
     "shadcn": "^4.0.0",
     "tailwindcss": "^4.2.0",
     "tw-animate-css": "1.3.3",
     "typescript": "5.7.3",
+    "typescript-eslint": "^8.65.0",
     "vitest": "^1.0.4"
   }
 }