Add base_url to native push notification payload

Include the server's base URL so native apps can identify which Fizzy
instance (SaaS or self-hosted) a notification originated from.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/models/notification/default_payload.rb

@@ -31,6 +31,10 @@ class Notification::DefaultPayload
     false
   end
 
+  def base_url
+    Rails.application.routes.url_helpers.root_url(**url_options.except(:script_name)).chomp("/")
+  end
+
   def avatar_url
     Rails.application.routes.url_helpers.user_avatar_url(notification.creator, **url_options)
   end