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>

mobile/package.json

@@ -6,7 +6,14 @@
     "start": "expo start",
     "android": "expo start --android",
     "ios": "expo start --ios",
-    "web": "expo start --web"
+    "web": "expo start --web",
+    "test": "jest"
+  },
+  "jest": {
+    "preset": "jest-expo",
+    "transformIgnorePatterns": [
+      "node_modules/(?!((jest-)?react-native|@react-native(-community)?|expo(nent)?|@expo(nent)?/.*|@expo-google-fonts/.*|react-navigation|@react-navigation/.*|@unimodules/.*|unimodules|sentry-expo|native-base|react-native-svg|axios))"
+    ]
   },
   "dependencies": {
     "@expo/vector-icons": "^14.0.0",
@@ -30,7 +37,10 @@
   },
   "devDependencies": {
     "@babel/core": "^7.25.0",
+    "@types/jest": "^30.0.0",
     "@types/react": "~19.0.0",
+    "jest": "^29.7.0",
+    "jest-expo": "~53.0.0",
     "typescript": "~5.8.0"
   },
   "private": true