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/docs/business-logic.md

@@ -214,14 +214,25 @@ Uses PKCS#7 signed `.pkpass` files and APNs push notifications.
 
 ## Cross-Store Redemption
 
-When `allow_cross_location_redemption` is enabled in merchant settings:
+The `allow_cross_location_redemption` merchant setting controls both card scoping and enrollment behavior:
 
-- Cards are scoped to the **merchant** (not individual stores)
+### When enabled (default)
+
+- **One card per customer per merchant** — enforced at the application layer
 - Customer can earn stamps at Store A and redeem at Store B
 - Each transaction records which `store_id` it occurred at
 - The `enrolled_at_store_id` field tracks where the customer first enrolled
+- If a customer tries to enroll at a second store, the system returns their existing card with a message showing all available locations
 
-When disabled, stamp/point operations are restricted to the enrollment store.
+### When disabled
+
+- **One card per customer per store** — each store under the merchant issues its own card
+- Stamp/point operations are restricted to the card's enrollment store
+- A customer can hold separate cards at different stores under the same merchant
+- Re-enrolling at the **same** store returns the existing card
+- Enrolling at a **different** store creates a new card scoped to that store
+
+**Database constraint:** Unique index on `(enrolled_at_store_id, customer_id)` prevents duplicate cards at the same store regardless of the cross-location setting.
 
 ## Enrollment Flow
 
@@ -229,21 +240,25 @@ When disabled, stamp/point operations are restricted to the enrollment store.
 
 Staff enrolls customer via terminal:
 1. Enter customer email (and optional name)
-2. System resolves or creates customer record
-3. Creates loyalty card with unique card number and QR code
-4. Creates `CARD_CREATED` transaction
-5. Awards welcome bonus points (if configured) via `WELCOME_BONUS` transaction
-6. Creates Google Wallet object and Apple Wallet serial
-7. Returns card details with "Add to Wallet" URLs
+2. System resolves customer — checks the current store first, then searches across all stores under the merchant for an existing cardholder with the same email
+3. If the customer already has a card (per-merchant or per-store, depending on the cross-location setting), raises `LoyaltyCardAlreadyExistsException`
+4. Otherwise creates loyalty card with unique card number and QR code
+5. Creates `CARD_CREATED` transaction
+6. Awards welcome bonus points (if configured) via `WELCOME_BONUS` transaction
+7. Creates Google Wallet object and Apple Wallet serial
+8. Returns card details with "Add to Wallet" URLs
 
 ### Self-Enrollment (Public)
 
 Customer enrolls via public page (if `allow_self_enrollment` enabled):
 1. Customer visits `/loyalty/join` page
-2. Enters email and name
-3. System creates customer + card
-4. Redirected to success page with card number
-5. Can add to Google/Apple Wallet from success page
+2. Enters email, name, and optional birthday
+3. System resolves customer (cross-store lookup for existing cardholders under the same merchant)
+4. If already enrolled: returns existing card with success page showing location info
+   - Cross-location enabled: "Your card works at all our locations" + store list
+   - Cross-location disabled: "Your card is registered at {original_store}"
+5. If new: creates customer + card, redirected to success page with card number
+6. Can add to Google/Apple Wallet from success page
 
 ## Scheduled Tasks