feat: add country filter to cities controller

- Introduce filtering of cities by selected country
- Retrieve current country using friendly find method
- Apply city filtering if country is provided in parameters

This change allows users to efficiently filter cities based on the
selected country, providing a more tailored experience and
encouraging better navigation through the list of cities.

app/controllers/cities_controller.rb

@@ -15,6 +15,11 @@ class CitiesController < ApplicationController
       @cities = @cities.by_region(@current_region.id) if @current_region
     end
 
+    if params[:country]
+      @current_country = Country.friendly.find(params[:country])
+      @cities = @cities.by_country(@current_country.id) if @current_country
+    end
+
     @cities = @cities.page(params[:page]).per(12)
 
     respond_to do |format|