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>

tests/integration/api/v1/admin/test_admin_users.py

@@ -367,14 +367,17 @@ class TestAdminAuthPlatformSelectionAPI:
 
         assert response.status_code == 403  # Access denied
 
-    def test_select_platform_as_super_admin_fails(
+    def test_select_platform_as_super_admin_succeeds(
         self, client, super_admin_headers, test_platform
     ):
-        """Test that super admin cannot select platform (they don't need to)."""
+        """Test that super admin can select platform context."""
         response = client.post(
             "/api/v1/admin/auth/select-platform",
             params={"platform_id": test_platform.id},
             headers=super_admin_headers,
         )
 
-        assert response.status_code == 401  # Super admins don't need this
+        assert response.status_code == 200
+        data = response.json()
+        assert data["platform_id"] == test_platform.id
+        assert data["platform_code"] == test_platform.code

tests/unit/utils/test_i18n.py

@@ -274,9 +274,14 @@ class TestLanguageResolution:
         assert result == "de"
 
     def test_resolve_storefront_enabled_filter(self):
-        """Test storefront respects enabled languages"""
+        """Test storefront enabled_languages filters automatic fallback but not explicit choices"""
+        # Explicit user choice (customer_preferred) should be respected
+        # even if not in store's enabled list
         result = resolve_storefront_language("de", None, None, None, ["en", "fr"])
-        # de is not in enabled list, should fallback
+        assert result == "de"
+
+        # But automatic fallback (browser, store default) should be filtered
+        result = resolve_storefront_language(None, None, None, "de", ["en", "fr"])
         assert result in ["en", "fr", DEFAULT_LANGUAGE]