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/static/storefront/js/loyalty-enroll.js

@@ -81,10 +81,21 @@ function customerLoyaltyEnroll() {
                         sessionStorage.setItem('loyalty_wallet_urls', JSON.stringify(response.wallet_urls));
                     }
 
-                    // Redirect to success page
+                    // Store enrollment context for the success page
+                    sessionStorage.setItem('loyalty_enroll_context', JSON.stringify({
+                        already_enrolled: response.already_enrolled || false,
+                        allow_cross_location: response.allow_cross_location ?? true,
+                        enrolled_at_store_name: response.enrolled_at_store_name || null,
+                        merchant_locations: response.merchant_locations || [],
+                    }));
+
+                    // Redirect to success page — pass already_enrolled in the
+                    // URL so the message survives page refreshes (sessionStorage
+                    // is supplementary for the location list).
                     const currentPath = window.location.pathname;
+                    const alreadyFlag = response.already_enrolled ? '&already=1' : '';
                     const successUrl = currentPath.replace(/\/join\/?$/, '/join/success') +
-                        '?card=' + encodeURIComponent(cardNumber);
+                        '?card=' + encodeURIComponent(cardNumber) + alreadyFlag;
                     window.location.href = successUrl;
                 }
             } catch (error) {