🌐(front) add calendar navigation translations

Add calendar.navigation.previous/next keys for toolbar
accessibility labels in EN, FR, and NL.

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

src/frontend/apps/calendars/src/features/i18n/translations.json

@@ -101,6 +101,10 @@
           "listYear": "Year list",
           "today": "Today"
         },
+        "navigation": {
+          "previous": "Previous",
+          "next": "Next"
+        },
         "event": {
           "createTitle": "Create event",
           "editTitle": "Edit event",
@@ -666,6 +670,10 @@
           "listYear": "Liste année",
           "today": "Aujourd'hui"
         },
+        "navigation": {
+          "previous": "Précédent",
+          "next": "Suivant"
+        },
         "event": {
           "createTitle": "Créer un événement",
           "editTitle": "Modifier l'événement",
@@ -978,6 +986,10 @@
           "listYear": "Jaar lijst",
           "today": "Vandaag"
         },
+        "navigation": {
+          "previous": "Vorige",
+          "next": "Volgende"
+        },
         "event": {
           "createTitle": "Evenement aanmaken",
           "editTitle": "Evenement bewerken",