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/en.json

@@ -141,6 +141,7 @@
     "didnt_receive_email": "Didn't receive the email? Check your spam folder or",
     "try_again": "try again",
     "remember_password": "Remember your password?",
+    "login_subtitle": "Welcome back to your account",
     "reset_password_subtitle": "Create new password",
     "reset_your_password": "Reset Your Password",
     "reset_password_form_desc": "Enter your new password below. Password must be at least 8 characters.",