Aggiunge traduzioni it/en/fr/de/es agli avvisi e toglie gli override Android/iOS.

Un avviso ha un testo per lingua; per differenziare le piattaforme si creano due avvisi. L'app riceve già il copy risolto da Accept-Language.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/db/migrate/20260820191000_remove_announcement_platform_copy.rb

@@ -0,0 +1,12 @@
+class RemoveAnnouncementPlatformCopy < ActiveRecord::Migration[7.2]
+  def change
+    remove_column :app_announcements, :android_title, :string
+    remove_column :app_announcements, :android_body, :text
+    remove_column :app_announcements, :android_action_url, :string
+    remove_column :app_announcements, :android_action_label, :string
+    remove_column :app_announcements, :ios_title, :string
+    remove_column :app_announcements, :ios_body, :text
+    remove_column :app_announcements, :ios_action_url, :string
+    remove_column :app_announcements, :ios_action_label, :string
+  end
+end