Migre eslint v8 → v9 (flat config), supprime les dépréciations

- eslint 8 → 9, @typescript-eslint/* → typescript-eslint (package unifié)
- Remplace .eslintrc.json par eslint.config.mjs (backend) et eslint.config.js (frontend)
- Ajoute eslint-plugin-react, eslint-plugin-react-hooks, globals (frontend)
- Supprime rimraf@3, @humanwhocodes/* qui étaient des deps transitives d'eslint v8

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -23,11 +23,14 @@
   "devDependencies": {
     "@types/react": "^18.2.28",
     "@types/react-dom": "^18.2.13",
-    "@typescript-eslint/eslint-plugin": "^6.7.5",
-    "@typescript-eslint/parser": "^6.7.5",
     "@vitejs/plugin-react": "^4.1.0",
     "autoprefixer": "^10.4.16",
-    "eslint": "^8.50.0",
+    "eslint": "^9.18.0",
+    "@eslint/js": "^9.18.0",
+    "typescript-eslint": "^8.22.0",
+    "eslint-plugin-react": "^7.37.0",
+    "eslint-plugin-react-hooks": "^5.1.0",
+    "globals": "^15.0.0",
     "postcss": "^8.4.31",
     "tailwindcss": "^3.3.3",
     "typescript": "^5.2.2",