Change PWA start URL from `/home` to `/` (#27377)

app/serializers/manifest_serializer.rb

@@ -16,11 +16,18 @@ class ManifestSerializer < ActiveModel::Serializer
     512
   ).freeze
 
-  attributes :name, :short_name,
+  attributes :id, :name, :short_name,
              :icons, :theme_color, :background_color,
              :display, :start_url, :scope,
              :share_target, :shortcuts
 
+  def id
+    # This is set to `/home` because that was the old value of `start_url` and
+    # thus the fallback ID computed by Chrome:
+    # https://developer.chrome.com/blog/pwa-manifest-id/
+    '/home'
+  end
+
   def name
     object.title
   end
@@ -53,7 +60,7 @@ class ManifestSerializer < ActiveModel::Serializer
   end
 
   def start_url
-    '/home'
+    '/'
   end
 
   def scope