✨(backend) serialize user language and timezone for frontend use

Add user language and timezone to serialized user data to enable frontend
customization. Allows backend email notifications to respect user's
localization preferences for improved communication relevance.

src/backend/core/api/serializers.py

@@ -4,6 +4,7 @@ from django.utils.translation import gettext_lazy as _
 
 from rest_framework import serializers
 from rest_framework.exceptions import PermissionDenied
+from timezone_field.rest_framework import TimeZoneSerializerField
 
 from core import models, utils
 
@@ -11,9 +12,11 @@ from core import models, utils
 class UserSerializer(serializers.ModelSerializer):
     """Serialize users."""
 
+    timezone = TimeZoneSerializerField()
+
     class Meta:
         model = models.User
-        fields = ["id", "email", "full_name", "short_name"]
+        fields = ["id", "email", "full_name", "short_name", "timezone", "language"]
         read_only_fields = ["id", "email", "full_name", "short_name"]