fix: update view count display in Ahoy Dashboard

- Change column header from '访问(日/周/年)' to '访问(DWMY)' to reflect
  updated metrics.
- Adjust metrics to include monthly view count alongside daily,
  weekly, and yearly counts.

This modification improves clarity by explicitly indicating the
metrics being displayed in the dashboard, thereby aiding users in
understanding the data better.

app/admin/ahoy_dashboard.rb

@@ -44,7 +44,7 @@ ActiveAdmin.register_page "Ahoy Dashboard" do
           table_for City.least_popular_active do
             column("ID") { |city| city.id }
             column("城市") { |city| link_to(city.name, admin_city_path(city)) }
-            column("访问(日/周/年)") { |city| "#{city.view_count(:day)}/#{city.view_count(:week)}/#{city.view_count(:year)}" }
+            column("访问(DWMY)") { |city| "#{city.view_count(:day)}/#{city.view_count(:week)}/#{city.view_count(:month)}/#{city.view_count(:year)}" }
             column("状态") { |city| status_tag(city.active? ? "活跃" : "停用") }
             column("所属区域") { |city| city.country.name+"/"+city.country.region.name }
             column("图像个数") { |city| city.weather_arts.count }
@@ -66,7 +66,7 @@ ActiveAdmin.register_page "Ahoy Dashboard" do
           table_for City.most_popular_inactive do
             column("ID") { |city| city.id }
             column("城市") { |city| link_to(city.name, admin_city_path(city)) }
-            column("访问(日/周/年)") { |city| "#{city.view_count(:day)}/#{city.view_count(:week)}/#{city.view_count(:year)}" }
+            column("访问(DWMY)") { |city| "#{city.view_count(:day)}/#{city.view_count(:week)}/#{city.view_count(:month)}/#{city.view_count(:year)}" }
             column("状态") { |city| status_tag(city.active? ? "活跃" : "停用") }
             column("所属区域") { |city| city.country.name+"/"+city.country.region.name }
             column("图像个数") { |city| city.weather_arts.count }