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>

app/modules/customers/templates/customers/storefront/login.html

@@ -56,7 +56,7 @@
                         <div class="text-6xl mb-4">🛒</div>
                         {% endif %}
                         <h2 class="text-2xl font-bold text-white mb-2">{{ store.name }}</h2>
-                        <p class="text-white opacity-90">Welcome back to your shopping experience</p>
+                        <p class="text-white opacity-90">{{ _("auth.login_subtitle") }}</p>
                     </div>
                 </div>