feat: enhance city localization and timezone handling

- Update localized_name method to provide a default value for
  missing translations
- Modify timezone display in city show view to show a message
  when the timezone is undefined

These changes improve user experience by ensuring that the city
localization falls back to the city name itself if a translation
is not found, and they handle potentially missing timezone data
more gracefully.

app/models/city.rb

@@ -84,7 +84,7 @@ class City < ApplicationRecord
   end
 
   def localized_name
-    I18n.t("cities.#{name.parameterize.underscore}")
+    I18n.t("cities.#{name.parameterize.underscore}", default: name)
   end
 
   def full_name

app/views/cities/show.html.erb

@@ -35,7 +35,7 @@
             <%= @city.country.name %>, <%= @city.region %>
           </div>
           <div class="badge badge-lg badge-secondary gap-2">
-            <%= Time.current.in_time_zone(@city.timezone).strftime("%Y-%m-%d %H:%M") %>
+            <%= @city.timezone.present? ? Time.current.in_time_zone(@city.timezone).strftime("%Y-%m-%d %H:%M") : "Timezone undefined" %>
           </div>
         </div>