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/dev_tools/locales/de.json

@@ -1,12 +1,17 @@
 {
-  "messages": {
-    "test_run_started": "Test run started...",
-    "failed_to_copy_code": "Failed to copy code",
-    "failed_to_copy_name": "Failed to copy name",
-    "added_to_cart": "Added to cart!",
-    "filters_applied": "Filters applied!",
-    "review_submitted_successfully": "Review submitted successfully!",
-    "thanks_for_your_feedback": "Thanks for your feedback!",
-    "code_copied_to_clipboard": "Code copied to clipboard!"
-  }
+    "messages": {
+        "test_run_started": "Test run started...",
+        "failed_to_copy_code": "Failed to copy code",
+        "failed_to_copy_name": "Failed to copy name",
+        "added_to_cart": "Added to cart!",
+        "filters_applied": "Filters applied!",
+        "review_submitted_successfully": "Review submitted successfully!",
+        "thanks_for_your_feedback": "Thanks for your feedback!",
+        "code_copied_to_clipboard": "Code copied to clipboard!"
+    },
+    "menu": {
+        "developer_tools": "Entwicklerwerkzeuge",
+        "components": "Komponenten",
+        "icons": "Icons"
+    }
 }

app/modules/dev_tools/locales/en.json

@@ -1,12 +1,17 @@
 {
-  "messages": {
-    "test_run_started": "Test run started...",
-    "failed_to_copy_code": "Failed to copy code",
-    "failed_to_copy_name": "Failed to copy name",
-    "added_to_cart": "Added to cart!",
-    "filters_applied": "Filters applied!",
-    "review_submitted_successfully": "Review submitted successfully!",
-    "thanks_for_your_feedback": "Thanks for your feedback!",
-    "code_copied_to_clipboard": "Code copied to clipboard!"
-  }
+    "messages": {
+        "test_run_started": "Test run started...",
+        "failed_to_copy_code": "Failed to copy code",
+        "failed_to_copy_name": "Failed to copy name",
+        "added_to_cart": "Added to cart!",
+        "filters_applied": "Filters applied!",
+        "review_submitted_successfully": "Review submitted successfully!",
+        "thanks_for_your_feedback": "Thanks for your feedback!",
+        "code_copied_to_clipboard": "Code copied to clipboard!"
+    },
+    "menu": {
+        "developer_tools": "Developer Tools",
+        "components": "Components",
+        "icons": "Icons"
+    }
 }

app/modules/dev_tools/locales/fr.json

@@ -1,12 +1,17 @@
 {
-  "messages": {
-    "test_run_started": "Test run started...",
-    "failed_to_copy_code": "Failed to copy code",
-    "failed_to_copy_name": "Failed to copy name",
-    "added_to_cart": "Added to cart!",
-    "filters_applied": "Filters applied!",
-    "review_submitted_successfully": "Review submitted successfully!",
-    "thanks_for_your_feedback": "Thanks for your feedback!",
-    "code_copied_to_clipboard": "Code copied to clipboard!"
-  }
+    "messages": {
+        "test_run_started": "Test run started...",
+        "failed_to_copy_code": "Failed to copy code",
+        "failed_to_copy_name": "Failed to copy name",
+        "added_to_cart": "Added to cart!",
+        "filters_applied": "Filters applied!",
+        "review_submitted_successfully": "Review submitted successfully!",
+        "thanks_for_your_feedback": "Thanks for your feedback!",
+        "code_copied_to_clipboard": "Code copied to clipboard!"
+    },
+    "menu": {
+        "developer_tools": "Outils de développement",
+        "components": "Composants",
+        "icons": "Icônes"
+    }
 }

app/modules/dev_tools/locales/lb.json

@@ -1,12 +1,17 @@
 {
-  "messages": {
-    "test_run_started": "Test run started...",
-    "failed_to_copy_code": "Failed to copy code",
-    "failed_to_copy_name": "Failed to copy name",
-    "added_to_cart": "Added to cart!",
-    "filters_applied": "Filters applied!",
-    "review_submitted_successfully": "Review submitted successfully!",
-    "thanks_for_your_feedback": "Thanks for your feedback!",
-    "code_copied_to_clipboard": "Code copied to clipboard!"
-  }
+    "messages": {
+        "test_run_started": "Test run started...",
+        "failed_to_copy_code": "Failed to copy code",
+        "failed_to_copy_name": "Failed to copy name",
+        "added_to_cart": "Added to cart!",
+        "filters_applied": "Filters applied!",
+        "review_submitted_successfully": "Review submitted successfully!",
+        "thanks_for_your_feedback": "Thanks for your feedback!",
+        "code_copied_to_clipboard": "Code copied to clipboard!"
+    },
+    "menu": {
+        "developer_tools": "Entwécklerwerkzäicher",
+        "components": "Komponenten",
+        "icons": "Icons"
+    }
 }