Use browser Accept-Language for login and onboarding locale (#768)

* Use Accept-Language for unauthenticated locale

* Add per-user locale overrides

* Fix test

* Use more than the top `accept-language` entry

* Localization of string

db/migrate/20260124180211_add_locale_to_users.rb

@@ -0,0 +1,6 @@
+class AddLocaleToUsers < ActiveRecord::Migration[7.2]
+  def change
+    add_column :users, :locale, :string, null: true, default: nil
+    add_index :users, :locale
+  end
+end