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/de.json

@@ -1,35 +1,41 @@
 {
-  "customers": {
-    "title": "Kunden",
-    "customer": "Kunde",
-    "add_customer": "Kunde hinzufügen",
-    "edit_customer": "Kunde bearbeiten",
-    "customer_name": "Kundenname",
-    "customer_email": "Kunden-E-Mail",
-    "customer_phone": "Kundentelefon",
-    "customer_number": "Kundennummer",
-    "first_name": "Vorname",
-    "last_name": "Nachname",
-    "merchant": "Firma",
-    "total_orders": "Bestellungen gesamt",
-    "total_spent": "Gesamtausgaben",
-    "last_order": "Letzte Bestellung",
-    "registered": "Registriert",
-    "no_customers": "Keine Kunden gefunden",
-    "search_customers": "Kunden suchen..."
-  },
-  "features": {
-    "customer_view": {
-      "name": "Kundenansicht",
-      "description": "Kundeninformationen anzeigen und verwalten"
+    "customers": {
+        "title": "Kunden",
+        "customer": "Kunde",
+        "add_customer": "Kunde hinzufügen",
+        "edit_customer": "Kunde bearbeiten",
+        "customer_name": "Kundenname",
+        "customer_email": "Kunden-E-Mail",
+        "customer_phone": "Kundentelefon",
+        "customer_number": "Kundennummer",
+        "first_name": "Vorname",
+        "last_name": "Nachname",
+        "merchant": "Firma",
+        "total_orders": "Bestellungen gesamt",
+        "total_spent": "Gesamtausgaben",
+        "last_order": "Letzte Bestellung",
+        "registered": "Registriert",
+        "no_customers": "Keine Kunden gefunden",
+        "search_customers": "Kunden suchen..."
     },
-    "customer_export": {
-      "name": "Kundenexport",
-      "description": "Kundendaten exportieren"
+    "features": {
+        "customer_view": {
+            "name": "Kundenansicht",
+            "description": "Kundeninformationen anzeigen und verwalten"
+        },
+        "customer_export": {
+            "name": "Kundenexport",
+            "description": "Kundendaten exportieren"
+        },
+        "customer_messaging": {
+            "name": "Kundennachrichten",
+            "description": "Nachrichten an Kunden senden"
+        }
     },
-    "customer_messaging": {
-      "name": "Kundennachrichten",
-      "description": "Nachrichten an Kunden senden"
+    "menu": {
+        "store_operations": "Shop-Betrieb",
+        "customers_section": "Kunden",
+        "customers": "Kunden",
+        "all_customers": "Alle Kunden"
     }
-  }
 }