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>

app/core/config.py

@@ -158,6 +158,13 @@ class Settings(BaseSettings):
     email_enabled: bool = True  # Set to False to disable all emails
     email_debug: bool = False  # Log emails instead of sending (for development)
 
+    # =============================================================================
+    # STOREFRONT DEFAULTS
+    # =============================================================================
+    # These can be overridden by AdminSetting in the database
+    default_storefront_locale: str = "fr-LU"  # Currency/number formatting locale
+    default_currency: str = "EUR"  # Default currency code
+
     # =============================================================================
     # DEMO/SEED DATA CONFIGURATION
     # =============================================================================