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/routes/api/storefront.py

@@ -19,6 +19,7 @@ from sqlalchemy.orm import Session
 from app.api.deps import get_current_customer_api
 from app.core.database import get_db
 from app.modules.customers.schemas import CustomerContext
+from app.modules.loyalty.exceptions import LoyaltyCardAlreadyExistsException
 from app.modules.loyalty.schemas import (
     CardEnrollRequest,
     CardResponse,
@@ -88,6 +89,7 @@ def self_enroll(
         customer_id=data.customer_id,
         email=data.email,
         store_id=store.id,
+        merchant_id=store.merchant_id,
         create_if_missing=True,
         customer_name=data.customer_name,
         customer_phone=data.customer_phone,
@@ -96,10 +98,45 @@ def self_enroll(
 
     logger.info(f"Self-enrollment for customer {customer_id} at store {store.subdomain}")
 
-    card = card_service.enroll_customer_for_store(db, customer_id, store.id)
+    # Build merchant context for the response (locations, cross-location flag)
+    settings = program_service.get_merchant_settings(db, store.merchant_id)
+    allow_cross_location = (
+        settings.allow_cross_location_redemption if settings else True
+    )
+    locations = program_service.get_merchant_locations(db, store.merchant_id)
+    location_list = [
+        {"id": loc.id, "name": loc.name}
+        for loc in locations
+    ]
+
+    already_enrolled = False
+    try:
+        card = card_service.enroll_customer_for_store(db, customer_id, store.id)
+    except LoyaltyCardAlreadyExistsException:
+        # Customer already has a card — return it instead of erroring out.
+        # For cross-location=true this is the normal re-enroll-at-another-store
+        # path; for cross-location=false this is a same-store re-enroll.
+        already_enrolled = True
+        if allow_cross_location:
+            card = card_service.get_card_by_customer_and_merchant(
+                db, customer_id, store.merchant_id
+            )
+        else:
+            card = card_service.get_card_by_customer_and_store(
+                db, customer_id, store.id
+            )
+
     program = card.program
     wallet_urls = wallet_service.get_add_to_wallet_urls(db, card)
 
+    # Resolve the name of the original enrollment store
+    enrolled_at_store_name = None
+    if card.enrolled_at_store_id:
+        for loc in locations:
+            if loc.id == card.enrolled_at_store_id:
+                enrolled_at_store_name = loc.name
+                break
+
     return {
         "card": CardResponse(
             id=card.id,
@@ -122,6 +159,10 @@ def self_enroll(
             has_apple_wallet=bool(card.apple_serial_number),
         ),
         "wallet_urls": wallet_urls,
+        "already_enrolled": already_enrolled,
+        "allow_cross_location": allow_cross_location,
+        "enrolled_at_store_name": enrolled_at_store_name,
+        "merchant_locations": location_list,
     }