fix(i18n): add missing menu translations and fix admin language resolution

Two issues caused the admin sidebar to show a mix of French and English:

1. Only 3 of 14 modules had "menu" translations in their locale files.
   When a key was missing, _translate_label() fell back to English Title
   Case from the key name — mixing with French from modules that had
   translations. Added menu sections to all 4 languages (en, fr, de, lb)
   across 13 modules.

2. The language middleware hardcoded admin to "en" ignoring user preference,
   while the menu API fell back to DEFAULT_LANGUAGE ("fr") when
   preferred_language was NULL. Fixed middleware to respect user's
   preferred_language and menu API to use middleware-resolved language
   as fallback.

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

app/modules/customers/locales/fr.json

@@ -1,35 +1,41 @@
 {
-  "customers": {
-    "title": "Clients",
-    "customer": "Client",
-    "add_customer": "Ajouter un client",
-    "edit_customer": "Modifier le client",
-    "customer_name": "Nom du client",
-    "customer_email": "E-mail du client",
-    "customer_phone": "Téléphone du client",
-    "customer_number": "Numéro client",
-    "first_name": "Prénom",
-    "last_name": "Nom",
-    "merchant": "Entreprise",
-    "total_orders": "Total des commandes",
-    "total_spent": "Total dépensé",
-    "last_order": "Dernière commande",
-    "registered": "Inscrit",
-    "no_customers": "Aucun client trouvé",
-    "search_customers": "Rechercher des clients..."
-  },
-  "features": {
-    "customer_view": {
-      "name": "Vue client",
-      "description": "Voir et gérer les informations clients"
+    "customers": {
+        "title": "Clients",
+        "customer": "Client",
+        "add_customer": "Ajouter un client",
+        "edit_customer": "Modifier le client",
+        "customer_name": "Nom du client",
+        "customer_email": "E-mail du client",
+        "customer_phone": "Téléphone du client",
+        "customer_number": "Numéro client",
+        "first_name": "Prénom",
+        "last_name": "Nom",
+        "merchant": "Entreprise",
+        "total_orders": "Total des commandes",
+        "total_spent": "Total dépensé",
+        "last_order": "Dernière commande",
+        "registered": "Inscrit",
+        "no_customers": "Aucun client trouvé",
+        "search_customers": "Rechercher des clients..."
     },
-    "customer_export": {
-      "name": "Export clients",
-      "description": "Exporter les données clients"
+    "features": {
+        "customer_view": {
+            "name": "Vue client",
+            "description": "Voir et gérer les informations clients"
+        },
+        "customer_export": {
+            "name": "Export clients",
+            "description": "Exporter les données clients"
+        },
+        "customer_messaging": {
+            "name": "Messagerie clients",
+            "description": "Envoyer des messages aux clients"
+        }
     },
-    "customer_messaging": {
-      "name": "Messagerie clients",
-      "description": "Envoyer des messages aux clients"
+    "menu": {
+        "store_operations": "Opérations du magasin",
+        "customers_section": "Clients",
+        "customers": "Clients",
+        "all_customers": "Tous les clients"
     }
-  }
 }