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/monitoring/locales/de.json

@@ -1 +1,12 @@
-{}
+{
+    "menu": {
+        "platform_health": "Plattform-Gesundheit",
+        "platform_monitoring": "Plattform-Überwachung",
+        "capacity_monitor": "Kapazitätsmonitor",
+        "testing_hub": "Test-Center",
+        "code_quality": "Code-Qualität",
+        "import_jobs": "Import-Aufträge",
+        "background_tasks": "Hintergrundaufgaben",
+        "application_logs": "Anwendungsprotokolle"
+    }
+}

app/modules/monitoring/locales/en.json

@@ -1 +1,12 @@
-{}
+{
+    "menu": {
+        "platform_health": "Platform Health",
+        "platform_monitoring": "Platform Monitoring",
+        "capacity_monitor": "Capacity Monitor",
+        "testing_hub": "Testing Hub",
+        "code_quality": "Code Quality",
+        "import_jobs": "Import Jobs",
+        "background_tasks": "Background Tasks",
+        "application_logs": "Application Logs"
+    }
+}

app/modules/monitoring/locales/fr.json

@@ -1 +1,12 @@
-{}
+{
+    "menu": {
+        "platform_health": "Santé de la plateforme",
+        "platform_monitoring": "Surveillance de la plateforme",
+        "capacity_monitor": "Moniteur de capacité",
+        "testing_hub": "Centre de tests",
+        "code_quality": "Qualité du code",
+        "import_jobs": "Tâches d'import",
+        "background_tasks": "Tâches de fond",
+        "application_logs": "Journaux applicatifs"
+    }
+}

app/modules/monitoring/locales/lb.json

@@ -1 +1,12 @@
-{}
+{
+    "menu": {
+        "platform_health": "Plattform-Gesondheet",
+        "platform_monitoring": "Plattform-Iwwerwaachung",
+        "capacity_monitor": "Kapazitéitsmonitor",
+        "testing_hub": "Test-Center",
+        "code_quality": "Code-Qualitéit",
+        "import_jobs": "Import-Aufträg",
+        "background_tasks": "Hannergrondaufgaben",
+        "application_logs": "Applikatiounsprotokoller"
+    }
+}