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>

middleware/language.py

@@ -61,9 +61,9 @@ class LanguageMiddleware(BaseHTTPMiddleware):
 
         # Resolve language based on frontend type
         if frontend_type == FrontendType.ADMIN:
-            # Admin dashboard: English only (for now)
-            # TODO: Implement admin language support later
-            language = "en"
+            # Admin dashboard: respect user's preferred language
+            user_preferred = self._get_user_language_from_token(request)
+            language = user_preferred or "en"
 
         elif frontend_type == FrontendType.STORE:
             # Store dashboard