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/tenancy/models/store.py

@@ -118,7 +118,7 @@ class Store(Base, TimestampMixin):
         String(5), nullable=False, default="fr"
     )  # Default language for customer-facing storefront
     storefront_languages = Column(
-        JSON, nullable=False, default=["fr", "de", "en"]
+        JSON, nullable=False, default=["fr", "de", "en", "lb"]
     )  # Array of enabled languages for storefront language selector
 
     # Currency/number formatting locale (e.g., 'fr-LU' = "29,99 EUR", 'en-GB' = "EUR29.99")