test: ajoute des tests automatisés frontend (Vitest) et mobile (Jest)

- Frontend Next.js : Vitest + Testing Library
  - Button (rendu, clic, état loading)
  - auth-store (setAuth, déconnexion, persistance token)
  - script `npm test`, config jsdom + alias @/
- Mobile Expo : Jest (preset jest-expo)
  - getApiError (erreur Axios, validation, erreur générique, sans réponse)
  - script `npm test`, transformIgnorePatterns adapté
- Cahier de tests : section « Outils de test automatisés » (PHPUnit + Vitest + Jest)
  et entrée 6.4 (PV de recette) au sommaire

Suites vertes : backend 15, frontend 6, mobile 4. tsc/lint/build OK, expo-doctor 18/18.

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

frontend/package.json

@@ -6,7 +6,8 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest run"
   },
   "dependencies": {
     "@tanstack/react-query": "^5.90.21",
@@ -22,12 +23,18 @@
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",
+    "@testing-library/dom": "^10.4.1",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
+    "@vitejs/plugin-react": "^6.0.2",
     "eslint": "^9",
     "eslint-config-next": "16.1.6",
+    "jsdom": "^29.1.1",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.1.9"
   }
 }