fix: sécuriser les identifiants Firebase avec variables d'environnement

- Déplacement des identifiants Firebase vers .env
- Ajout de .env au .gitignore
- Installation de expo-constants
- Validation automatique des variables au démarrage
- Mise à jour de .env.example avec EXPO_PUBLIC_ prefix
- Mise à jour du README avec instructions .env

BREAKING CHANGE: Les identifiants Firebase doivent maintenant être configurés dans .env

package-lock.json

@@ -14,6 +14,7 @@
         "@react-navigation/native": "^7.1.18",
         "@react-navigation/stack": "^7.5.0",
         "expo": "~54.0.18",
+        "expo-constants": "~18.0.10",
         "expo-image-picker": "^17.0.8",
         "expo-status-bar": "~3.0.8",
         "firebase": "^12.4.0",
@@ -5875,6 +5876,20 @@
         }
       }
     },
+    "node_modules/expo-constants": {
+      "version": "18.0.10",
+      "resolved": "https://registry.npmjs.org/expo-constants/-/expo-constants-18.0.10.tgz",
+      "integrity": "sha512-Rhtv+X974k0Cahmvx6p7ER5+pNhBC0XbP1lRviL2J1Xl4sT2FBaIuIxF/0I0CbhOsySf0ksqc5caFweAy9Ewiw==",
+      "license": "MIT",
+      "dependencies": {
+        "@expo/config": "~12.0.10",
+        "@expo/env": "~2.0.7"
+      },
+      "peerDependencies": {
+        "expo": "*",
+        "react-native": "*"
+      }
+    },
     "node_modules/expo-image-loader": {
       "version": "6.0.0",
       "resolved": "https://registry.npmjs.org/expo-image-loader/-/expo-image-loader-6.0.0.tgz",
@@ -6317,20 +6332,6 @@
         "react-native": "*"
       }
     },
-    "node_modules/expo/node_modules/expo-constants": {
-      "version": "18.0.10",
-      "resolved": "https://registry.npmjs.org/expo-constants/-/expo-constants-18.0.10.tgz",
-      "integrity": "sha512-Rhtv+X974k0Cahmvx6p7ER5+pNhBC0XbP1lRviL2J1Xl4sT2FBaIuIxF/0I0CbhOsySf0ksqc5caFweAy9Ewiw==",
-      "license": "MIT",
-      "dependencies": {
-        "@expo/config": "~12.0.10",
-        "@expo/env": "~2.0.7"
-      },
-      "peerDependencies": {
-        "expo": "*",
-        "react-native": "*"
-      }
-    },
     "node_modules/expo/node_modules/expo-file-system": {
       "version": "19.0.17",
       "resolved": "https://registry.npmjs.org/expo-file-system/-/expo-file-system-19.0.17.tgz",