feat: multi-module improvements across merchant, store, i18n, and customer systems

- Fix platform-grouped merchant sidebar menu with core items at root level
- Add merchant store management (detail page, create store, team page)
- Fix store settings 500 error by removing dead stripe/API tab
- Move onboarding translations to module-owned locale files
- Fix onboarding banner i18n with server-side rendering + context inheritance
- Refactor login language selectors to use languageSelector() function (LANG-002)
- Move HTTPException handling to global exception handler in merchant routes (API-003)
- Add language selector to all login pages and portal headers
- Fix customer module: drop order stats from customer model, add to orders module
- Fix admin menu config visibility for super admin platform context
- Fix storefront auth and layout issues
- Add missing i18n translations for onboarding steps (en/fr/de/lb)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/customers/tests/unit/test_customer_model.py

@@ -49,8 +49,6 @@ class TestCustomerModel:
 
         assert customer.is_active is True  # Default
         assert customer.marketing_consent is False  # Default
-        assert customer.total_orders == 0  # Default
-        assert customer.total_spent == 0  # Default
 
     def test_customer_full_name_property(self, db, test_store):
         """Test Customer full_name computed property."""