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/analytics/locales/en.json

@@ -1,17 +1,20 @@
 {
-  "analytics": {
-    "page_title": "Analytics",
-    "dashboard_title": "Analytics Dashboard",
-    "dashboard_subtitle": "View your store performance metrics and insights",
-    "period_7d": "Last 7 days",
-    "period_30d": "Last 30 days",
-    "period_90d": "Last 90 days",
-    "period_1y": "Last year",
-    "imports_count": "Imports",
-    "products_added": "Products Added",
-    "inventory_locations": "Inventory Locations",
-    "data_since": "Data since",
-    "loading": "Loading analytics...",
-    "error_loading": "Failed to load analytics data"
-  }
+    "analytics": {
+        "page_title": "Analytics",
+        "dashboard_title": "Analytics Dashboard",
+        "dashboard_subtitle": "View your store performance metrics and insights",
+        "period_7d": "Last 7 days",
+        "period_30d": "Last 30 days",
+        "period_90d": "Last 90 days",
+        "period_1y": "Last year",
+        "imports_count": "Imports",
+        "products_added": "Products Added",
+        "inventory_locations": "Inventory Locations",
+        "data_since": "Data since",
+        "loading": "Loading analytics...",
+        "error_loading": "Failed to load analytics data"
+    },
+    "menu": {
+        "analytics": "Analytics"
+    }
 }