fix(loyalty): cross-store enrollment, card scoping, i18n flicker

Fix duplicate card creation when the same email enrolls at different
stores under the same merchant, and implement cross-location-aware
enrollment behavior.

- Cross-location enabled (default): one card per customer per merchant.
  Re-enrolling at another store returns the existing card with a
  "works at all our locations" message + store list.
- Cross-location disabled: one card per customer per store. Enrolling
  at a different store creates a separate card for that store.

Changes:
- Migration loyalty_004: replace (merchant_id, customer_id) unique
  index with (enrolled_at_store_id, customer_id). Per-merchant
  uniqueness enforced at application layer when cross-location enabled.
- card_service.resolve_customer_id: cross-store email lookup via
  merchant_id param to find existing cardholders at other stores.
- card_service.enroll_customer: branch duplicate check on
  allow_cross_location_redemption setting.
- card_service.search_card_for_store: cross-store email search when
  cross-location enabled so staff at store2 can find cards from store1.
- card_service.get_card_by_customer_and_store: new service method.
- storefront enrollment: catch LoyaltyCardAlreadyExistsException,
  return existing card with already_enrolled flag, locations, and
  cross-location context. Server-rendered i18n via Jinja2 tojson.
- enroll-success.html: conditional cross-store/single-store messaging,
  server-rendered translations and context, i18n_modules block added.
- dashboard.html, history.html: replace $t() with server-side _() to
  fix i18n flicker across all storefront templates.
- Fix device-mobile icon → phone icon.
- 4 new i18n keys in 4 locales (en, fr, de, lb).
- Docs: updated data-model, business-logic, production-launch-plan,
  user-journeys with cross-location behavior and E2E test checklist.
- 12 new unit tests + 3 new integration tests (334 total pass).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/models/loyalty_card.py

@@ -255,11 +255,14 @@ class LoyaltyCard(Base, TimestampMixin, SoftDeleteMixin):
         cascade="all, delete-orphan",
     )
 
-    # Indexes - one card per customer per merchant
+    # Indexes
+    # One card per customer per store (always enforced at DB level).
+    # Per-merchant uniqueness (when cross-location is enabled) is enforced
+    # by the application layer in enroll_customer().
     __table_args__ = (
-        Index("idx_loyalty_card_merchant_customer", "merchant_id", "customer_id", unique=True),
+        Index("idx_loyalty_card_store_customer", "enrolled_at_store_id", "customer_id", unique=True),
+        Index("idx_loyalty_card_merchant_customer", "merchant_id", "customer_id"),
         Index("idx_loyalty_card_merchant_active", "merchant_id", "is_active"),
-        Index("idx_loyalty_card_customer_program", "customer_id", "program_id", unique=True),
         # Balances must never go negative — guards against direct SQL writes
         # bypassing the service layer's clamping logic.
         CheckConstraint("points_balance >= 0", name="ck_loyalty_cards_points_balance_nonneg"),