Add creator initials and avatar color to push notification payload

Move avatar_background_color logic from helper to User::Avatar concern
so it can be accessed from models. Include creator_id, creator_initials,
and creator_avatar_color in native push notifications for local avatar
rendering on iOS.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

saas/app/models/notification/push_target/native.rb

@@ -34,7 +34,10 @@ class Notification::PushTarget::Native < Notification::PushTarget
           avatar_url: payload.avatar_url,
           card_id: card&.id,
           card_title: card&.title,
+          creator_id: notification.creator.id,
           creator_name: notification.creator.name,
+          creator_initials: notification.creator.initials,
+          creator_avatar_color: notification.creator.avatar_background_color,
           category: payload.category
         )
         .new(