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/marketplace/locales/de.json

@@ -91,5 +91,15 @@
         "manage_integration_desc": "Marktplatz-Integration konfigurieren",
         "sync_products": "Produkte synchronisieren",
         "sync_products_desc": "Produkte mit dem Marktplatz synchronisieren"
+    },
+    "onboarding": {
+        "connect_api": {
+            "title": "Letzshop-API verbinden",
+            "description": "Konfigurieren Sie Ihren API-Schlüssel für die Synchronisation mit Letzshop"
+        },
+        "import_products": {
+            "title": "Produkte importieren",
+            "description": "Importieren Sie Ihren Produktkatalog von Letzshop"
+        }
     }
 }