fix(loyalty): conditional subtitle on enrollment success page

When re-enrolling (already a member), the success page showed two
contradicting messages stacked:
  Title:    "Vous êtes déjà membre !"          (correct, conditional)
  Subtitle: "Vous êtes maintenant membre..."   (wrong — static)

The title was already x-text-conditional based on
enrollContext.already_enrolled, but the subtitle was a server-side
{{ _('success.message') }} so it always rendered the "you're now a
member" copy regardless of branch.

Make the subtitle conditional the same way:
- new i18n key already_enrolled_message in en/fr/de/lb
  ("Welcome back — your card is ready whenever you are." and
  locale-appropriate equivalents)
- expose success_message + already_enrolled_message in i18nStrings
- subtitle becomes x-text="already_enrolled ? msg2 : msg1"

Found during Test 2 round 2 — cross-store re-enrollment at
FASHIONOUTLET with the email from Test 1.

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

app/modules/loyalty/locales/en.json

@@ -137,6 +137,7 @@
             "back_to_home": "Back to Home"
         },
         "already_enrolled_title": "You're already a member!",
+        "already_enrolled_message": "Welcome back — your card is ready whenever you are.",
         "cross_location_message": "Your card works at all our locations:",
         "single_location_message": "Your card is registered at {store_name}",
         "available_locations": "Use your card at all our locations:",