fix(login-i18n): translate the "Welcome back ..." subtitle on customer login

Last unilingual string in customers/storefront/login.html was the
"Welcome back to your shopping experience" line on the branding side.
Was flagged as scope-creep-skip during the original Test 5 i18n sweep
since the user only complained about reset-password and dashboard
pages; surfaced now that the 401 redirect lands users on /account/login
mid-French-session.

Replaced with {{ _("auth.login_subtitle") }} and added the key to all
4 core locales (en/fr/de/lb). Reworded slightly from "shopping
experience" since loyalty storefronts have no catalog — same reasoning
as the auth.continue_shopping → auth.back_to_home rename earlier.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/locales/fr.json

@@ -141,6 +141,7 @@
     "didnt_receive_email": "Vous n'avez pas reçu l'e-mail ? Vérifiez votre dossier spam ou",
     "try_again": "réessayez",
     "remember_password": "Vous vous souvenez de votre mot de passe ?",
+    "login_subtitle": "Heureux de vous revoir",
     "reset_password_subtitle": "Créer un nouveau mot de passe",
     "reset_your_password": "Réinitialisez votre mot de passe",
     "reset_password_form_desc": "Saisissez votre nouveau mot de passe ci-dessous. Il doit comporter au moins 8 caractères.",