feat: add configurable currency locale and fix vendor JS init

Currency Locale Configuration:
- Add platform-level storefront settings (locale, currency)
- Create PlatformSettingsService with resolution chain:
  vendor → AdminSetting → environment → hardcoded fallback
- Add storefront_locale nullable field to Vendor model
- Update shop routes to resolve and pass locale to templates
- Add window.SHOP_CONFIG for frontend JavaScript access
- Centralize formatPrice() in shop-layout.js using SHOP_CONFIG
- Remove local formatPrice functions from shop templates

Vendor JS Bug Fix:
- Fix vendorCode being null on all vendor pages
- Root cause: page components overriding init() without calling parent
- Add parent init call to 14 vendor JS files
- Add JS-013 architecture rule to prevent future regressions
- Validator now checks vendor JS files for parent init pattern

Files changed:
- New: app/services/platform_settings_service.py
- New: alembic/versions/s7a8b9c0d1e2_add_storefront_locale_to_vendors.py
- Modified: 14 vendor JS files, shop templates, validation scripts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/shop/js/shop-layout.js

@@ -219,12 +219,15 @@ function shopLayoutData() {
             }, 3000);
         },
 
-        // Format currency
-        formatPrice(price) {
-            return new Intl.NumberFormat('en-US', {
+        // Format currency using configured locale
+        formatPrice(amount) {
+            if (!amount && amount !== 0) return '';
+            const locale = window.SHOP_CONFIG?.locale || 'fr-LU';
+            const currency = window.SHOP_CONFIG?.currency || 'EUR';
+            return new Intl.NumberFormat(locale, {
                 style: 'currency',
-                currency: 'USD'
-            }).format(price);
+                currency: currency
+            }).format(amount);
         },
 
         // Format date