Add sidebar i18n support and fix language selector visibility

Extended the internationalization system to support sidebar navigation menu with French/English translations. Fixed language selector dropdown styling to improve text visibility with proper contrast (gray background instead of transparent).

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

html/lang/fr.json

@@ -49,5 +49,16 @@
     "deviceName": "Nom de l'appareil",
     "deviceID": "ID de l'appareil",
     "modemVersion": "Version du modem"
+  },
+  "sidebar": {
+    "home": "Accueil",
+    "sensors": "Capteurs",
+    "database": "Base de données",
+    "modem4g": "Modem 4G",
+    "wifi": "WIFI",
+    "logs": "Logs",
+    "map": "Carte",
+    "terminal": "Terminal",
+    "admin": "Admin"
   }
 }