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/data-model.md

@@ -120,14 +120,21 @@ Merchant-wide loyalty program configuration. One program per merchant, shared ac
 
 ### LoyaltyCard
 
-Customer loyalty card linking a customer to a merchant's program. One card per customer per merchant.
+Customer loyalty card linking a customer to a merchant's program.
+
+**Card uniqueness depends on the `allow_cross_location_redemption` merchant setting:**
+
+- **Cross-location enabled (default):** One card per customer per merchant. The application layer enforces this by checking all stores under the merchant before creating a card. Re-enrolling at another store returns the existing card.
+- **Cross-location disabled:** One card per customer per store. A customer can hold separate cards at different stores under the same merchant, each scoped to its enrollment store.
+
+**Database constraint:** Unique index on `(enrolled_at_store_id, customer_id)` — always enforced. The per-merchant uniqueness (cross-location enabled) is enforced at the application layer in `card_service.enroll_customer`.
 
 | Field | Type | Description |
 |-------|------|-------------|
 | `merchant_id` | FK | Links to program's merchant |
 | `customer_id` | FK | Card owner |
 | `program_id` | FK | Associated program |
-| `enrolled_at_store_id` | FK | Store where customer enrolled |
+| `enrolled_at_store_id` | FK | Store where customer enrolled (part of unique constraint) |
 | `card_number` | String (unique) | Formatted XXXX-XXXX-XXXX |
 | `qr_code_data` | String (unique) | URL-safe token for QR codes |
 | `stamp_count` | Integer | Current stamp count |